The surface area of a rectangular prism = base perimeter * height + 2 * area of the base.

The dimensions of the rectangular prism base are 3 by 8, so since opposite sides of a rectangle are equal the perimeter = 2*3 + 2*8 = 22

The area of the base is length * width = 3*8 = 24

Let the height of the prism, which is the unknown "w"

The surface area equations becomes

22 * w + + 2 * 24 = 246, which you can solve for w.