partial derivatives

Hi all.
What command do I give mathematica to solve the below. Thank you!

Simply:

D[(3 x - 4 y)/(x^2 + 4 y), y, x]

or did I misunderstand your question?

That works. Thank you.
I initially typed the same thing, but for some reason, Mathematica was giving me error messages. I restarted it, and now it works.