How do you evaluate ##b^2-4ac## for a=-1, b=-5, and c=2?

Plug in your numbers.

##b^2 - 4ac##

##{(b= - 5), (a = -1), (c = 2) :}##

Plug in these values

##(-5)^2 + 4 (-1)(2)##

Since

##-5 * (-5) = 25##

you have

##25+4(-1)(2)##

Since

##4 * (-1) = -4##

##-4 * 2 = -8##

you end up with

##25 - 8 = 17##