Answer
$a. 0$
$b. 3a - 12$
$c. -3x - 12$
$d. 3x + 3h - 12$
Work Step by Step
$f(x) = 3x - 12$
$a.$ $f(4)$
$f(4) = 3(4) - 12$
$f(4) = 12 - 12$
$f(4) = 0$
$b.$ $f(a)$
$f(a) = 3(a) - 12$
$f(a) = 3a - 12$
$c. f(-x)$
$f(-x) = 3(-x) - 12$
$f(-x) = -3x - 12$
$d. f(x + h)$
$f(x + h) = 3(x + h) - 12$
$f(x + h) = 3x + 3h - 12$