Given how unlikely it is to get a useful explanation here, I suggest just using www.wolframalpha.com for cheating instead, you would've gotten Pathak's solution with for instance "dy/dx of y= -x/(a^2-x^2)" or "derivative of y = -x/(a^2-x^2)". An added plus is that it won't judge you for being too lazy to execute the straightforward derivation algorithm yourself. And also that it's very likely to be correct, so you don't have to rely on majority voting.