Chapter 8 Rational Functions - 8.1 Model Inverse and Joint Variation - Guided Practice for Examples 5 and 6 - Page 554: 12

See below.

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: $56=a(-6)(-4)=24a\\a=\frac{8}{3}$. Thus the model is: $z=\frac{8}{3}xy$. Then plugging in $x=-2,y=5$: $z=\frac{8}{3}(-2)(5)=\frac{-80}{3}$