Composition functions are functions in which the input goes in and the output of the first function becomes the input for the second. You can evaluate these in two ways.
1) You can simply input the x value into the first function. After solving for it's value, you can input into the second one. The output is the composite functions answer.
2) If they only want a new function or you want to do it a different way, you have to put the first function (the whole thing) in for x in every place. You can then simplify.