Explanation:
the easiest way might be the approach to calculate the area of the whole rectangle and then subtract the area of the triangle.
the area of the whole rectangle is
18×16 = 288 m²
the area of a triangle is
baseline × height / 2
in a case like this (right-angled triangle) we can use one leg as baseline, and the other leg (90°) as height.
so, the height is the same as the width of the rectangle : 16 m.
how long is the baseline ?
it is on the 18 m length of the rectangle, and there are two 3 m "gaps", one left, and one right.
so, what's left for the baseline of the triangle itself ?
baseline = 18 - 3 - 3 = 12 m
therefore the area of the triangle is
12×16/2 = 12×8 = 96 m²
so, the total area of the shaded region is
288 - 96 = 192 m²