Answer
$$-\dfrac{y \sin xy}{x \sin xy+\cos y}$$
Work Step by Step
We have: $$ \cos x=1+\sin y$$
and $F(x,y)=\cos (xy)=1-\sin y=0$
Now, $$F_x=-y \sin xy \\ F_y= -x \sin (xy) -\cos y$$
Apply equation: $\dfrac{dy}{dx}=-\dfrac{F_x}{F_y}$
Thus, we have:
$$\dfrac{dy}{dx}=-\dfrac{-y \sin xy}{-x \sin (xy) -\cos y} \\ =-\dfrac{y \sin xy}{x \sin xy+\cos y}$$