If $ \sqrt{1-y^2}-\sqrt{1-x^2}=a(x-y),\normalsize$ then find $\large\frac{dy}{dx}$

Question

$\begin{array}{1 1}(a)\;\sqrt{\large\frac{1-x^2}{1-y^2}}\\(b)\; -\sqrt{\large\frac{1-x^2}{1-y^2}}\\(c)\;\sqrt{\large\frac{1-y^2}{1-x^2}}\\(d)\;-\sqrt{\large\frac{1-y^2}{1-x^2}}\end{array}$