Explanation:
well, it is a rectangle, where an isoceles right-angled triangle is cut off in the top left corner, but that triangle is added to the top right corner.
so, the area is the same as the regular rectangle.
we know, the area of a rectangle is
length × width
now, we know the length for sure (10 m).
what is the width ?
we know the part of the width until the triangle is 5 m.
and we know the leg length of the triangle is 4 m.
the width is simply the sum of the basic part and the leg length
5 + 4 = 9 m
so the area of all this is
10 × 9 = 90 m²