Solve for $k$ when the equation has equal roots

Question

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, $b^2-4ac$.

Answer 1

$b^2-4ac=4^2-4(2)(-k)=16+8k=0$ imply $k=-2$

Answer 2

$\textbf{Another approach:}$ \begin{align} 2x^2 + 4x - k &= 0\\ x^2 +4x-\frac{k}{2}&= 0 \\ (x+1)^2-1 &=\frac{k}{2} \\ 0 &=\frac{k}{2}+1 \\ k &=-2 \end{align}