How do I integrate admob in my android app in android studio 3.0

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Java

Android

Am trying to integrate AdMob into my android application but its not working out for me, I have added the bellow maven directive to my all project section:

maven {
            url "https://maven.google.com"
        }

I have added the bellow dependency to the dependency section:

implementation 'com.google.android.gms:play-services-ads:16.0.0'

And that was the beginning of my problem, its throwing two errors, it underlined in red

implementation 'com.android.support:appcompat-v7:26.0.0-beta1'

error saying

all com.android.support libraries must use the exact same version specification

and

implementation 'com.google.android.gms:play-services-ads:16.0.0'

error saying

all gms/firebase libraries must use the exact same version specification

Please I need help...

What I have tried:

Manifest File
<?xml version="1.0" encoding="utf-8"?>
<manifest xmlns:android="http://schemas.android.com/apk/res/android"
package="com.example.gns.projecttopicsfinder">
<uses-permission android:name="android.permission.INTERNET"/>
<uses-permission android:name="android.permission.ACCESS_NETWORK_STATE"/>

<application
android:allowBackup="true"
android:icon="@mipmap/ic_launcher"
android:label="Project Topics Finder"
android:roundIcon="@mipmap/ic_launcher_round"
android:supportsRtl="true"
android:theme="@style/AppTheme">
<activity android:name="com.example.gns.projecttopicsfinder.MainActivity">
<intent-filter>
<action android:name="android.intent.action.MAIN" />

<category android:name="android.intent.category.LAUNCHER" />


<activity android:name="com.example.gns.projecttopicsfinder.CompSc" />
<activity android:name="com.example.gns.projecttopicsfinder.CompEng" />
<activity android:name="com.example.gns.projecttopicsfinder.MassComm" />
<activity android:name="com.example.gns.projecttopicsfinder.Civil" />
<activity android:name="com.example.gns.projecttopicsfinder.Mechanical" />
<activity android:name="com.example.gns.projecttopicsfinder.PublicAdmin" />
<activity android:name="com.example.gns.projecttopicsfinder.Psychology" />
<activity android:name="com.example.gns.projecttopicsfinder.Soil" />
<activity android:name="com.example.gns.projecttopicsfinder.Sociology" />
<activity android:name="com.example.gns.projecttopicsfinder.Banking">





All Project Section

// Top-level build file where you can add configuration options common to all sub-projects/modules.

buildscript {

repositories {
google()
jcenter()
}
dependencies {
classpath 'com.android.tools.build:gradle:3.0.0'


// NOTE: Do not place your application dependencies here; they belong
// in the individual module build.gradle files
}
}

allprojects {
repositories {
google()
jcenter()
maven {
url "https://maven.google.com"
}
}
}

task clean(type: Delete) {
delete rootProject.buildDir
}



Dependencies


apply plugin: 'com.android.application'

android {
compileSdkVersion 26
defaultConfig {
applicationId "com.example.gns.projecttopicsfinder"
minSdkVersion 21
targetSdkVersion 26
versionCode 1
versionName "1.0"
testInstrumentationRunner "android.support.test.runner.AndroidJUnitRunner"
}
buildTypes {
release {
minifyEnabled false
proguardFiles getDefaultProguardFile('proguard-android.txt'), 'proguard-rules.pro'
}
}
}

dependencies {
implementation fileTree(dir: 'libs', include: ['*.jar'])
implementation 'com.android.support:appcompat-v7:26.0.0-beta1'
implementation 'com.android.support.constraint:constraint-layout:1.0.2'
testImplementation 'junit:junit:4.12'
androidTestImplementation 'com.android.support.test:runner:0.5'
androidTestImplementation 'com.android.support.test.espresso:espresso-core:2.2.2'
implementation 'com.google.android.gms:play-services-ads:16.0.0'
}