check the picture below.
the base of it is one of the triangles, and thus, the sides are just the 3 rectangles.
notice all you have is a 3x5 rectangle, a 5x5 one and a 4x5 rectangle.
get their area, sum them up, and that's the lateral, sides, area of the triangular prism.
(3 · 5) + (5 · 5) + (4 · 5)
