UPDATE: Solved thanks to turkyhundt and jimbo
The mathematical question is as follows:
Calculate the value of $k$ for which $2x^2 + 4x - k = 0$ has equal roots.
My working solves it to equal $-2$, but if we then put this back into the formula this is, of course, impossible. I use the discriminate of the quadratic equation, $b^2-4ac$.
$b^2-4ac=4^2-4(2)(-k)=16+8k=0$ imply $k=-2$