How do you find the sum of two functions?

Without knowing what the functions are, you have to leave it as a sum. For example, f(x)+g(x) is as far as you can get, if you don't know what f(x) or g(x) are.

If you are told f(x)=x+5, and g(x)=2x-1, you can plug in to get x+5+2x-1, which just equals 3x+4.