Answer
$b^{2}-4ac=0$
Work Step by Step
$a=1,b=-6,c=9$
$ b^{2}-4ac\qquad$ ...substitute $1$ for $a,\ -6$ for $b$ and $9$ for $c$.
$=(-6)^{2}-4\cdot 1\cdot 9\qquad$ ...evaluate $(-6)^{2}$
$=36-4\cdot 1\cdot 9\qquad$ ...multiply before subtracting
$=36-36$
$=0$