Is y^2 = 4 - x^2 the same as:
y= 2 - x
y= (4 - x^2)/y
If you're wanting to solve for y, you will need to take the square root of both sides of the equation.
And that's about it.
Now, if you had something like: , you could get to because the right side of the equation was a perfect square. That wasn't the case in your example.