Explanation:
the total area is the sum of the area of the left 3×3 square, the bottom 11×3 rectangle and the 11×3 triangle.
the area of a square and rectangle is
length × width
the area of a triangle is
baseline × height / 2
the area of the square is 3×3 = 9 ft²
the area of the rectangle is 11×3 = 33 ft²
the area of the triangle is 11×3/2 = 16.5 ft²
because for a right-angled triangle we can use directly the legs as baseline and height.
so, the total area is
9 + 33 + 16.5 = 58.5 ft²