Algebra 1: Common Core (15th Edition)

$81$
$x^2+18x+c$ Compare it with $ax^2+bx+c$, we have $b=18$ For perfect-square trinormal add $(\frac{b}{2})^2$Plug $b=18$ to complete the square. $c=(\frac{81}{2})^2=(9)^2=81$