Explanation:
(f○g)(x) = f(g(x))
so, we use the expression of g(x) as the "x" in f(x).
(f○g)(x) = f(g(x)) = 3(x² + 3) - 1 = 3x² + 9 - 1 = 3x² + 8
so,
(f○g)(3) = 3×3² + 8 = 27 + 8 = 35
check :
g(3) = 3² + 3 = 9 + 3 = 12
f(g(3)) = f(12) = 3×12 - 1 = 36 - 1 = 35
correct.