Answer
$ŷ=3891-296x_1+0.552x_2$
Work Step by Step
In MINITAB, enter the Slump values in C1, the 7-Day psi values in C2 and the 28-Day psi values in C3.
Select Stats -> Regression -> Regression -> Fit Regression Model
Enter C3 in "Responses" and C1 C2 in "Continuous Predictors"
The least-squares regression line will be shown in "Regression Equation", where C3 is $ŷ$ (Slump), C1 is $x_1$ (7-Day psi) and C2 is $x_2$ (28-Day psi).
$ŷ=3891-296x_1+0.552x_2$
