This is the given problem I have:

Use the Chain Rule (or the Invariance Property of diﬀerentials) to ﬁnd the ﬁrst order partial derivatives (i.e. ∂g/∂s and ∂g/∂t)

of g(s,t) := f(s,u(s,t),v(s,t)), where

u(s,t) := st,

v(s,t) := s + t.

The answer should be expressed in terms of s and t only.

I'm completely stuck on this problem, due to the Equation itself. Dont really know how to do it, anyone that can help me?