Answer
{$-2,-1,3,4$}
Work Step by Step
Since, $u^2-11u+24=0$
Factorize the expression as follows: $(u-8)(u-3)=0$
or, $u=${$3,8$}
Replace $u$ with $x^2-2x$, we have
$x^2-2x=3 \implies x^2-2x-3=0$
or, $(x-3)(x+1)=0$
$x=${$-1,3$}
and $x^2-2x=8 \implies x^2-2x-8=0$
or, $(x-4)(x+2)=0$
$x=${$-2,4$}
Hence, our solution set is $x=${$-2,-1,3,4$}
