# What is the vertex form of y= x^2-21x ?

${\left(x - \frac{21}{2}\right)}^{2} - \frac{441}{4}$
${x}^{2} - 21 x$
$= {x}^{2} - 21 x + {\left(- \frac{21}{2}\right)}^{2} - {\left(- \frac{21}{2}\right)}^{2}$
$= {\left(x - \frac{21}{2}\right)}^{2} - \frac{441}{4}$