Answer
$3(1-3x)(1+3x)$
Work Step by Step
Factor out $3$ to obtain:
$=3(1-9x^2)
\\=3[1^2-(3x)^2]$
RECALL:
$a^2-b^2=(a-b)(a+b)$
Use the formula above with $a=1$ and $b=3x$ to factor the binomial and obtain:
$=3(1-3x)(1+3x)$