[update, thanks to Richard]
There's nothing wrong with your program.
Actually there is an error in your code, as well spotted by Richard
Should be instead
result = a(number1,number2)
I would have written it this way
print('this program will give you the result of multiplication, division and addition of two numbers')
result = num1 + num2
number1 = ask_num( 'please enter the first no.')
number2 = ask_num( 'please enter the second no.')
result = add(number1, number2)
print('the result is ' + str(result))
but it's just matter of taste.