How to you solve for the square units?

Call this "surface area," which is measured in square units.

The top and bottom are both squares, each with surface area 5 units.  Thus, the total surface area of the top and bottom is (5 units)^2, or 25 units^2.
Each side has a width of 5 units and a height of 3(5 units) = 5*15 units^2.
There are four sides.  Thus, the total area of the four sides is 4(75 units^2)=300 units^2, or 300 units^2.

Adding together the areas of the top and bottom and the four sides:
25 units^2 + 300 units^2 = 325 units^2.

Think of it as five or above give it a shove. change the 5 or above to the right to a zero and add one to the number on the left.