Basically, you try to find a function contained inside a function if that makes sense. For example in question 23, g(x) itself is a function = . Same with f(x) = . In this case, when we try to fo f(g(x)), we are basically replacing x in the f(x) function by the equation in g(x). Hope this makes sense.
The total space needed is 600 feet. Because 5 tables times 72 feet is 360 and 5 tables times 48 feet a part is 249. You add them and it comes out to be 600. It should be worked out like this 5(72+48)