If x + y = xy
then dy/dx is?
You can solve the equation for y, but it is simpler to use implicit differentiation (which I presume is what they want here.)
<-- Take the derivative.
Now, if you need a form of dy/dx explicitly in terms of y, then
You can either just take the derivative of this (not that hard to do) or plug this expression for y into dy/dx and simplify.
-Dan