Answer
See below.
Work Step by Step
$k^2-3k=28\\k^2-3k-28=0\\(k+4)(k-7)=0$
So $k=-4$ or $k=7$.
If $k=7$, then $\frac{x+3}{x-4}=7\\x+3=7(x-4)\\x+3=7x-28\\31=6x\\x=\frac{31}{6}$.
If $k=-4$, then $\frac{x+3}{x-4}=-4\\x+3=-4(x-4)=x+3=-4x+16\\5x=19\\x=3.8$.
