When we have a function such as h(x) = 2x, and we want to find the value of h at a given x value, we plug in the given number for x. For example, h(3) = 2*3 = 6.
We do the same thing with g(f(7)). In this case we plug 7 into f(x) for x, then plug the result of f(7) into g(x) for x.
