Triangular Prisms

A right triangular prism's net is two triangles and three rectangles. The triangles are the ends. Each rectangle is one side of the triangle stretched along the prism's length.

Here the ends are right triangles with legs 4 and 3, so the two ends together make $\frac{2\cdot 1}{2}\cdot 4\cdot 3 = 12$. The rectangles are 4, 3 and 5 wide and all 6 long, so they make $\left(4+3+5\right)\cdot 6 = 72$. The surface area is $12+72 = 84$.

The rule is Surface area of a prism: the two ends plus the perimeter of an end times the length. This time every side of the triangle counts, the longest one too, because each one is a face.