Here are two examples. Use them to work your problems.
For the first one, you simply plug in the function g for x in function f. B is the same process for different functions.
C is a little trickier. What it is asking you to do is plug in the function h for x in function g. Once you get that, you then plug in function f into all x's in the g o h composite function. Then simplify. D is the same thing, but the functions are being plugged in in a different order.