Consider that the area of the shaded region is equal to the area of the rectangle minus the area of the white triangle.
Then, you have:
shaded area = A1 - A2
where,
A1: area of the rectangle = (7 ft)(12 ft) = 84 ft^2
A2: area of the triangle = (7ft - 3ft)(12 ft)/2 = (4 ft)(12 ft)/2 = 24 ft^2
Then, for the shaded region you obtain:
shaded area = 84 ft^2 - 24 ft^2 = 60 ft^2
Hence, the area of the shaded region is 60 ft^2
