Answer
$b^{2}-4ac=-108$
Work Step by Step
$a=-4,b=2,c=-7$
$ b^{2}-4ac\qquad$ ...substitute $-4$ for $a,\ 2$ for $b$ and $-7$ for $c$.
$=(2)^{2}-4\cdot(-4)\cdot(-7)\qquad$ ...evaluate $(2)^{2}$
$=4-4\cdot(-4)\cdot(-7)\qquad$ ...multiply before subtracting
$=4-112$
$=-108$
