Did you apply the distributive law when you multiplied the first equation.

4*(1-x^2)^(1/2) + 2*x(1-x^2)^(-1/2)*(-2*x)

Then, your common denominator was correct. But you would end up with this,

numerator: (4*(1-x^2) - 4*x^2)

denominator: ((1-x^2)^(1/2))

I don't know how to do all the code, but if you apply the distributive law you should end up with what is in green; if unclear let me know.