$x^2 + 18x \:+ \:? =$
$(x\: + \:?)^2$

$−32 = x^2 + 18x$

$−32 + 81 = x^2 + 18x + 81$

$49 = (x + 9)^2$

$\pm7=x+9$

$x = -16$ or $–2$

What needs to be added to both sides of the equation so that the left side can be rewritten as $(x + 6)^2$?