The Following Estimated Regression Equation Has Been Proposed to Predict $\hat { y } = 40 - 3 x _ { 1 } + 12 x _ { 2 } + 10 x _ { 3 }$

The following estimated regression equation has been proposed to predict monthly sales at a shoe store.
​ $\hat { y } = 40 - 3 x _ { 1 } + 12 x _ { 2 } + 10 x _ { 3 }$
where
$\begin{array} { l } x _ { 1 } = \text { competitor's previous month's sales (in } \$ 1000 \mathrm {~s} \text { ) } \\x _ { 2 } = \text { store's previous month's sales (in } \$ 1000 \mathrm {~s} \text { ) } \\x _ { 3 } = 1 \text { if radio advertising was used; } 0 \text { otherwise } \\\widehat { y } = \text { estimated sales (in } \$ 1000 \text { s) }\end{array}$
a.
Predict sales (in dollars) for the shoe store if the competitor's previous month's sales were $9000, the store's previous month's sales were $30,000, and no radio advertisements were run.
b.
Predict sales (in dollars) for the shoe store if the competitor's previous month's sales were $9000, the store's previous month's sales were $30,000, and 10 radio advertisements were run.