Answer
See below.
Work Step by Step
$k^2-k=12\\k^2-k-12=0\\(k+3)(k-4)=0$
So $k=-3$ or $k=4$.
If $k=4$, then $\frac{x+3}{x-3}=4\\x+3=4(x-3)=x+3=4x-12\\15=3x\\x=5$.
If $k=-3$, then $\frac{x+3}{x-3}=-3\\x+3=-3(x-3)=x+3=-3x+9\\4x=6\\x=1.5$.