Explanation:
1)
imagine the trapezoid standing upright (90°) turned.
then the top and bottom lines are parallel, and the 15 side is with its double right angles the height of the trapezoid.
in general, the area of such a trapezoid is
(top + bottom)/2 × height
in our case that is
(3 + 4)/2 × 15 = 7/2 × 15 = 3.5 × 15 = 52.5 units²
2)
this is basically the sum of the lower rectangle and the upper trapezoid.
the area of the lower rectangle is
58×15 = 870 mm²
the area of the upper trapezoid is (the same formula as before)
(47 + 58)/2 × (21 - 15) = 105/2 × 6 = 52.5 × 6 = 315 mm²
so, the total area is
870 + 315 = 1,185 mm² = 1,185.0 mm²