The problem lies within your if statement. You're using a single equals sign when you should be using two.
if (operation == 1)
Two equal signs is a comparison operator. It checks to see if one value equals another value. In your case, its checking if operation equals 1. But, this only solves half the problem. Operation is a string and you need to compare it to a string.
if (operation == "1")
I hope this helps!
