Find $x-\sqrt{7x}$ given that $x - \sqrt{\frac7x}=8$ 
If $ x - \sqrt{\frac{7}{x}}=8$ then $x-\sqrt{7x}=\text{?}$ 

I used some ways, but couldn't get the right form :) by the way, the answer is $1$.
Thanks in advance.

Let $x=7y^2,$ where $y>0$.
Thus, $$7y^2-\frac{1}{y}=8$$ or
$$7y^3-8y-1=0$$ or
$$7y^3+7y^2-7y^2-7y-y-1=0$$ or
$$(y+1)(7y^2-7y-1)=0.$$
Thus,
$$x-\sqrt{7x}=7y^2-7y=1.$$
Done!
