Given the functions a(x) = 3x − 12 and b(x) = x − 9, solve a[b(x)] and select the correct answer below.
a[b(x)] = 3x2 − 21
a[b(x)] = 3x2 − 39
a[b(x)] = 3x − 21
a[b(x)] = 3x − 39