Answer
$36$; $(x+6)^2$
Work Step by Step
To complete the square, add a term that is the square of half the coefficient of $x$ in the middle term. Then factor by trial and error.
$x^2+12x$
$(\frac{12}{2})^2=6^2=36$
$x^2+12x+36$
$(x+6)^2$