Answer
$k=84$
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:
$\sqrt{36x^2}=6$
Find $b$ by taking the principal square root of the last term to obtain:
$b=\sqrt{49}=7$
Thus, the middle term of the given trinomial is:
$=2(6x)(7)\\
=84x$
Thus, $k=84$.
