Explanation:
1. here we just use a standard formula. the area of a triangle is
A = baseline × height / 2
so,
20×8/2 = 10×8 = 80 m²
2.
this shape consists of a rectangle and a triangle.
we need to calculate the areas of both and then simply add them.
the area of the rectangle is 6×5 = 30 cm²
the triangle is defined by the height (5 cm) of the overall shape (which is then for that right-angled triangle also its own height), the extension of the shape baseline (14 - 6 = 8 m) and the longer connection of these two lines (but we don't need to calculate this).
the area of that triangle is then 8×5/2 = 4×5 = 20 cm²
together that is 30+20 = 50 cm²
3.
this prism (or simply "box") has 6 surface areas with 3 pairs of equally sized areas.
top-bottom : 15×12 (2 times)
front-back : 12×6 (2 times)
left-right : 15×6 (2 times)
so, we have
15×12×2 + 12×6×2 + 15×6×2 = 360 + 144 + 180 = 684 in²