#### Answer

$4x$

#### Work Step by Step

RECALL:
The perfect square trinomial $a^2-2ab+b^2$ can be factored using the formula:
$a^2-2ab+b^2=(a-b)^2$
The given trinomial can be written as:
$=(4x0^2-2(4x)(3)+3^2$
This trinomial above is a perfect square trinomial which can be factored using the formula given above.
Factor using the formula above with $a=4x$ and $b=3$ to obtain:
$=(4x-3)^2$
Thus, the missing expression in the given statement is:
$4x$