I'm assuming you want the total surface area.
This is a trapezoidal prism. The bases are the parallel faces running horizontal (parallel to the ground) and they are congruent trapezoids. The lateral sides are rectangles.
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Lateral Faces:
The back face (hidden from view) is a rectangle that is 12 inches by 3 inches, so it has an area of 12*3 = 36 sq inches
The front slant faces on the left and right are each 3*5 = 15 sq inches in area
The front center lateral face has area of 6*3 = 18 sq inches
The total lateral surface area is 15+18+15+36 = 84 sq inches
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The bases are each the same trapezoid. The trapezoid has two parallel sides of 12 and 6 inches. Call this b1 and b2. The height of the trapezoid is h = 4. Imagine the trapezoid is laid flat in a 2D perspective instead of a 3D one.
The area of one trapezoid is...
A = h*(b1+b2)/2
A = 4*(12+6)/2
A = 4*(18)/2
A = 72/2
A = 36
So the area of both base faces combined is 2*36 = 72
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The total surface area is then found by adding the total lateral surface area (84) and the total base area (72) to get 84+72 = 156
Final Answer: 156 square inches
