The surface area is the area of the base plus the areas of the four sides.
The area of the base is 6*6=36. The area of each one of the four sides is (1/2)*x*6 = 3x The area of the four sides combined is 3x * 4 = 12x. The total surface area is 12x + 36. The question says that it is 84. So, 84 = 12x + 36 Solve: 84-36 = 12x 48 = 12x 48/12 = x x = 4
