Answer
$\frac{81}{4}$; $(x-\frac{9}{2})^2$
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.
$x^2-9x$
$(-\frac{9}{2})^2=\frac{81}{4}$
$x^2-9x+\frac{81}{4}$
$(x-\frac{9}{2})^2$
