how to clear the text fields

I "see" 4 textboxes:
d1 - input value,
q1 - input value,
un1 - input value,
amt1 - result.

To solve your problem, just add new button and move the body of q1_TextChanged() procedure into Button1_Click() procedure.
To clear textboxes, copy code below and paste it before Me.Refresh() command:

d1.text = ""
q1.text = ""
un1.text = ""

Now, your textboxes are clear.


****************[EDIT]Second solution - step by step with custom Class and Interface[EDIT]****************

1) Create new (windows application) project
2) Add 4 lables and 4 texboxes and change them names as follows: TxtVal1, TxtVal2, TxtVal3, TxtVal4
3) Add 1 button (Name: BtnCalc)
4) Add new Interface[^] (menu Project->Add new item->Interface) and save it as "ITestCalc.vb". We need it to correspond with between class and form.
5) Copy and paste code below:

Public Interface ITestCalc
    Function Calc() As Integer
    Function Calc(ByVal _d1 As Integer, ByVal _q1 As Integer, ByVal _un1 As Integer) As Integer
    Property D1() As Integer
    Property Q1() As Integer
    Property Un1() As Integer
End Interface

6) Add new Class[^] to the project (Menu Project->Add new item->Class) and save it as "TestCalc.vb"
7) Copy code below and paste it into your class:

Public Class TestCalc
    Implements ITestCalc

    Private d1 As Integer = 0
    Private q1 As Integer = 0
    Private un1 As Integer = 0

    'default constructor
    Public Sub New()
        'do nothing
    End Sub

    'constructor with input values
    Public Sub New(ByVal _d1 As Integer, ByVal _q1 As Integer, ByVal _un1 As Integer)
        d1 = _d1
        q1 = _q1
        un1 = _un1
    End Sub


    Public Function Calc() As Integer Implements ITestCalc.Calc
        Dim retVal As Integer = 0

        retVal = d1 * q1 * (un1 / 30)

        Return retVal
    End Function

    Function Calc(ByVal _d1 As Integer, ByVal _q1 As Integer, ByVal _un1 As Integer) As Integer Implements ITestCalc.Calc
        d1 = _d1
        q1 = _q1
        un1 = _un1
        Return Calc()
    End Function

    Public Property D11() As Integer Implements ITestCalc.D1
        Get
            Return d1
        End Get
        Set(ByVal value As Integer)
            d1 = value
        End Set
    End Property

    Public Property Q11() As Integer Implements ITestCalc.Q1
        Get
            Return q1
        End Get
        Set(ByVal value As Integer)
            q1 = value
        End Set
    End Property

    Public Property Un11() As Integer Implements ITestCalc.Un1
        Get
            Return un1
        End Get
        Set(ByVal value As Integer)
            un1 = value
        End Set
    End Property

    Protected Overrides Sub Finalize()
        MyBase.Finalize()
    End Sub
End Class

8) In the form (class):

Public Class Form1
    'declare variable
    Private oCalc As ITestCalc = New TestCalc()

    Private Sub BtnCalc_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles BtnCalc.Click
        Try
            oCalc.D1 = Int(Me.TxtVal1.Text)
            oCalc.Q1 = Int(Me.TxtVal2.Text)
            oCalc.Un1 = Int(Me.TxtVal3.Text)
            Me.TxtResult.Text = oCalc.Calc().ToString
            'or
            'Me.TxtResult.Text = oCalc.Calc(Int(Me.TxtVal1.Text), Int(Me.TxtVal2.Text), Int(Me.TxtVal3.Text)).ToString

        Catch ex As InvalidCastException
            MsgBox(ex.Message & vbCr & vbCr & _
                "Enter correct values!", MsgBoxStyle.Exclamation, "Error!")

        Catch ex As Exception
            MsgBox(ex.Message, MsgBoxStyle.Exclamation, "Error!")
        End Try
    End Sub

    Protected Overrides Sub Finalize()
        oCalc = Nothing
        MyBase.Finalize()
    End Sub
End Class

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