Diagonal = √2 * side = 3√2 *√2 = 3*2 = 6
ANSWER
The correct answer is

EXPLANATION
The expression given to us is,

We plug in

. That means we substitute 12 wherever we see x to obtain,

We simplify the parenthesis first to obtain,





Hence the expression evaluates to
Surface area is what it seems to be. All of the area of the solid's faces together. Like a cube, just find the areas of all of the squares that make the cube then add them together.