Answer
$S=10x^2+4x-8$
For $x=6$:
$S=376$
Work Step by Step
$V=2x^3+x^2-8x-4$
$V=x^2(2x+1)-4(2x+1)$
$V=(x^2-4)(2x+1)=(x^2-2^2)(2x+1)$
$V=(x+2)(x-2)(2x+1)$
So, the edges of the cube are: $(x+2),~(x-2)~and~(2x+1)$
The surface area:
$S=2(x+2)(x-2)+2(x+2)(2x+1)+2(x-2)(2x+1)=$
$S=2(x^2-4)+2(2x^2+x+4x+2)+2(2x^2+x-4x-2)=$
$S=10x^2+4x-8$
For $x=6$:
$S=10(6)^2+4(6)-8=360+24-8=376$
