Answer
$\frac{25}{4}$; $(x+\frac{5}{2})^2$
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.
$x^2+5x$
$(\frac{5}{2})^2=\frac{25}{4}$
$x^2+5x+\frac{25}{4}$
$(x+\frac{5}{2})^2$