Use the multiple regression equation to predict the $y$ -values for the values of the independent variables.
The volume (in cubic feet) of a black cherry tree can be modeled by the equation $$\hat{y}=-52.2+0.3 x_{1}+4.5 x_{2}$$ where $x_{1}$ is the tree's height (in feet) and $x_{2}$ is the tree's diameter (in inches). (Source: Journal of the Royal Statistical Society)
(a) $x_{1}=70, x_{2}=8.6$
(b) $x_{1}=65, x_{2}=11.0$
(c) $x_{1}=83, x_{2}=17.6$
(d) $x_{1}=87, x_{2}=19.6$