solve this equation for $x$ : $y=x-6\sqrt{x}$ solve for $x$ this equation :
$$y=x-6\sqrt{x}$$
I've tried raising everything to the power of two but it doesn't work 
$x$ shouldn't have two values.

$$\begin{align} y = x-6\sqrt x & \iff (y-x) = -6\sqrt x \\ \\&\implies  y^2 - 2xy + x^2 = 36x\\ \\ &\iff x^2 - (2y - 36)x + y^2 = 0\end{align}$$
Now you can solve for $x$ using the quadratic equation (treat $y$ as a constant), but you'll want to check whether both solutions work, because by squaring, we may have introduced a solution that is not valid in the original equation.
$$x = \dfrac{(2y-36) \pm \sqrt{(2y - 36)^2 - 4y^2}}{2}$$
Now, it's just simplifying, and remember to check both solutions: Recall that $x\geq 0$ or else $\sqrt x$ is not defined.
