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