Answer
After removing Curb Weight:
$ŷ=26.80+2.15x_1-0.0503x_3$
After removing Engine Size:
$ŷ=28.72-0.03040x_3$
Work Step by Step
Let's remove Curb Weight at first because it has the highest P-value:
In MINITAB, enter the Engine Size values in C1, the Horsepower values in C3 and the Miles per Gallon values in C4.
Select Stats -> Regression -> Regression -> Fit Regression Model
Enter C4 in "Responses" and C1 C3 in "Continuous Predictors"
The least-squares regression line will be shown in "Regression Equation", where C4 is $ŷ$ (Miles per Gallon), C1 is $x_1$ (Engine Size) and C3 is $x_3$ (Horsepower).
$ŷ=26.80+2.15x_1-0.0503x_3$
But, for the Engine Size:
$t_0=1.00$ with a P-value $=0.341\ltα=0.05$. Do not reject the null hypothesis.
So, let's remove the Engine Size:
In MINITAB, enter the the Horsepower values in C3 and the Miles per Gallon values in C4.
Select Stats -> Regression -> Regression -> Fit Regression Model
Enter C4 in "Responses" and C3 in "Continuous Predictors"
The least-squares regression line will be shown in "Regression Equation", where C4 is $ŷ$ (Miles per Gallon) and C3 is $x_3$ (Horsepower).
$ŷ=28.72-0.03040x_3$
P-value $=0.004\ltα=0.05$
