Answer
See below.
Work Step by Step
The general equation for inverse variation is: $y=\frac{a}{x}$ where $a$ is some constant. Then plugging in the given values for $x,y$ we get: $18=\frac{a}{\frac{5}{2}}\\a=45$.
Thus the model is: $y=\frac{45}{x}$. Then plugging in $x=-3$: $y=\frac{45}{-3}=-15$
