X= -4√x+12
What are you trying to do? Solve for x? Then:
$\displaystyle x = -4\sqrt{x} + 12 \Rightarrow x - 12 = -4 \sqrt{x} \Rightarrow (x - 12)^2 = 16x \Rightarrow x^2 - 40x + 144 = 0$.
One of the solutions to this equation is the solution to $\displaystyle x = -4\sqrt{x} + 12$, the other is not (it's an extraneous solution and is in fact the solution to $\displaystyle x = 4\sqrt{x} + 12$).