Answer
$12\sqrt 2$
Work Step by Step
$S: z=4-x$
$dS=\sqrt 2$
Substitute
$z=4-x$
(x,y) $\approx$D
D is the projection of the surface on the xy plane
$\sqrt 2[4][4(3)-(3)^2]$
$12\sqrt 2$
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