Answer
$f(3x) = -18x^2+4$
Work Step by Step
RECALL:
$f(a)$ represents the value of the function $f$ when $x=a$.
Substitute $3x$ into $x$ in the function $f$ to obtain:
$f(3x) = 4-2(3x)^2
\\f(3x) = 4-2(9x^2)
\\f(3x) = 4-18x^2
\\f(3x) = -18x^2+4$