Explanation:
the surface area is the sum of all areas of the individual outside faces/sides.
so, what do we see here, what outside sides do we have here ?
of course, we have to assume that this is a regular object (left and right sides are the same).
2 right-angled triangles (left and right).
1 rectangle in the back.
1 rectangle at the bottom.
1 times rectangle in the front.
the area of a right-angled triangle is
leg1 × leg2 / 2
in our case that is
6 × 2.5 /2 = 3 × 2.5 = 7.5 mm²
we have 2 of these triangles :
2×7.5 = 15 mm²
the area of the rectangle in the back is
8 × 6 = 48 mm²
the area of the rectangle at the bottom is
8 × 2.5 = 20 mm²
the area of the rectangle in the front is
8 × 6.5 = 52 mm²
the surface of the object is the sum of all that :
15 + 48 + 20 + 52 = 135 mm²