Answer
$k=12$
Work Step by Step
Recall:
$a^2+2ab+b^2=(a+b)^2$
Find $a$ by taking the principal square root of the first term to obain:
$\sqrt{9x^2}=3x$
Find $b$ by taking the principal square root of the last term to obtain:
$b=\sqrt{4}=2$
Thus, the middle term of the given trinomial is:
$=2(3x)(2)\\
=12x$
Thus, $k=12$.