Explanation:
the area of the parallelogram is
baseline × height.
we know the baseline (15 cm), because as parallelogram it must have 2 pays off equal sides. so, the bottom side must be 15 cm too.
15×height = 390
height = 390/15 = 26 cm
now, the height of the triangle on the right is the same height.
and the baseline of this triangle is 25 - 15 = 10 cm.
the area of a triangle is
baseline × height / 2
so, in our case
10 × 26 / 2 = 10×13 = 130 cm²
therefore the area of the trapezium is the area of the parallelogram plus the area of the triangle :
390 + 130 = 520 cm²