The area of the lower-left trapezoid is
.. A₁ = (1/2)(b₁+b₂)h
.. A₁ = (1/2)(36 ft +18 ft)(12 ft)
.. A₁ = 324 ft²
The area of the upper-right trapezoid is
.. A₂ = (1/2)(b₁+b₂)h
.. A₂ = (1/2)(24 ft +12 ft)(18 ft)
.. A₂ = 324 ft²
The area of the floor plan is
.. A = A₁ + A₂
.. A = 324 ft² + 324 ft²
.. A = 648 ft²
The area of the floor plan is 648 square feet.
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Check
It can also be computed as (36 ft)(24 ft) - (18 ft)(12 ft) = 648 ft².