Answer
$ŷ=4207.4~psi$
Work Step by Step
$ŷ=3891-296x_1+0.552x_2$
For a sample of concrete with the slump equal to 3.5 inches and the 7-day strength equal to 2450 psi, that is, $x_1=3.5$ and $x_2=2450$:
$ŷ=3891-296\times3.5+0.552\times2450=4207.4$
