Answer
See below.
Work Step by Step
The general equation for joint variation is: $z=axy$ where $a$ is some constant. Then plugging in the given values for $z,x,y$ we get: $7=a(1)(2)=2a\\a=3.5$.
Thus the model is: $z=3.5xy$. Then plugging in $x=-2,y=5$: $z=3.5(-2)(5)=-35$
