Explanation:
f(g(x)) simply means that g(x) is embedded in f(x) as the input value for f(x).
f(g(x)) = 3(7x + 6) - 1 = 21x + 18 - 1 = 21x + 17
c. is the right answer
g(f(x)) is the other way around. f(x) is now the input value for g(x).
g(f(x)) = 7(3x - 1) + 6 = 21x - 7 + 6 = 21x - 1
a. is the right answer