The Hidden Side First

The surface area needs every side of the triangle end, but a figure often gives only the two legs of a right triangle. The third side is not missing: the Pythagorean theorem fixes it.

Here the legs are 3 and 4, so the longest side is $\sqrt{{3}^{2}+{4}^{2}} = 5$. Now the surface area: the ends make $3\cdot 4 = 12$ together, the rectangles make $\left(3+4+5\right)\cdot 2 = 24$, and the total is 36.

On the board that is two steps. Tap the unlabelled side and pick Pythagorean theorem, then tap the prism and pick Surface area of a prism.