If you meant the sum of f(x) + g(x), then the answers below are correct. If you meant the
composition of f(x) and g(x), that is f(g(x)) or f ° g, then substitute g(x) for x in f(x):
f ° g = 5 - (-9x + 7) = 9x - 2
Note that composition does not obey the associative law, that is f ° g does not equal g ° f