Answer
x=16
Work Step by Step
Solving by setting the factors equal to zero, we find:
$$-x=-\sqrt{x}-12 \\ \left(-x+12\right)^2=\left(-\sqrt{x}\right)^2 \\ x^2-24x+144=x \\ x=16$$
Note, the solution x=9 does not result in a true statement when plugged back in.