Explanation:
we can split that figure into 2 shapes:
a 5 cm ×3 cm rectangle on the right
a right-angled triangle on the left with leg lengths of 4 cm (horizontal) and 3 cm (vertical).
the area of the trapezium is the sum of the 2 sub-areas.
the rectangle area is
length × width = 5×3 = 15 cm²
the triangle area is
baseline × height / 2
for a right-angled triangle we can use the 2 legs as baseline and height :
4×3/2 = 2×3 = 6 cm²
so the area of the trapezium is
15 + 6 = 21 cm²