I would first isolate that square root:

and then square both sides.

[tex]\frac{Q^2}{C^2A^2}= \frac{2gh}{1- \frac{A^2}{x^2}}

Multiply both numerator and denominator, on the right, by

Get rid of the fractions by multiplying both sides by the denominators, and

Add to both sides

Factor [tex]A^2[/quote] out of the right side

Divide both sides by

Finally, take the square root of both sides