Answer
$x^2+24x+144$
Work Step by Step
Using $(a+b)^2=a^2+2ab+b^2$ or the square of a binomial, the given expression, $
(x+12)(x+12)
$, is equivalent to
\begin{array}{l}\require{cancel}
(x+12)^2
\\\\=
(x)^2+2(x)(12)+(12)^2
\\\\=
x^2+24x+144
.\end{array}