Question about Implicit Differentiation

My question is when do you multiply by y' or $\displaystyle \frac d{dx}$ when trying to get the derivative?

I know that in this problem $\displaystyle x^3 + y^3 = 25$ that you can do it $\displaystyle 3x + 3y (y') = 0$

In more complicated equations I don't know when to use it. For example in this problem

$\displaystyle sin(x) + cos(y) = sin(x)cos(y)$

I don't know where y' belongs. Can anyone help me find out when I have to multiply by y'. I know you are supposed to when you're using the chain rule but I don't know how to do it in that type of problem.

-FP!