Given the following estimated linear model
$$
\hat{y}=10+2 x_1+12 x_2+8 x_3
$$
a. What is the change in $\hat{y}$ when $x_1$ increases by 4 ?
b. What is the change in $\hat{y}$ when $x_3$ increases by 1 ?
c. What is the change in $\hat{y}$ when $x_2$ increases by 2 ?