Chain rule with a given gradient

Let $F(s,t)=f(st,s-t)$, with $f=f(u,v)$ differentiable and

$$\nabla f(2,1)=(3,-2).$$

Compute $\frac{\partial F}{\partial s}$ and $\frac{\partial F}{\partial t}$ at the point $(s,t)=(2,1)$.