Can you please supply us with your definition of help?
We can't be sure what sort of help you need, because you didn't tell us. But one thing you may be missing is that when they write h(y), they are telling you that there is no x in that term; as far as x is concerned, h(y) is a constant. For example, f(x,y) might be 2x2 + 7y5, so that g(x) = 2x2 and h(y) = 7y5. The first is a function only of x, and the second is a function only of y.