\underbrace{x^3y}_{x^2 \cdot xy}\underbrace{\left(ydx+xdy\right)}_{d\left(xy\right)}=\underbrace{2xydx+x^2dy}_{d\left(x^2y\right)=d\left(x \cdot xy\right)=d\left(x\right) \cdot xy + x d\left(xy\right)}