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

A (x) = 3x - 12

b (x) = x - 9

Substitute x of a (x) equation with the value of b (x).

a[b (x) ] = 3[b (x) ] - 12

a[b (x) ] = 3 (x-9) - 12

a[b (x) ] = 3x - 27 - 12

a[b (x) ] = 3x - 39

a[b (x) ] = 3 (x - 13)