1. **Problem statement:** Find the surface area of a triangular prism with triangle side lengths 9 in., 12 in., and 15 in., and height (depth) 12 in. 2. **Formula for surface area of a triangular prism:** $$\text{Surface Area} = 2 \times \text{Area of triangular base} + \text{Perimeter of triangle} \times \text{height}$$ 3. **Calculate the area of the triangular base:** The triangle sides are 9, 12, and 15 inches. This is a right triangle (since $9^2 + 12^2 = 81 + 144 = 225 = 15^2$). Area of right triangle = $\frac{1}{2} \times \text{base} \times \text{height} = \frac{1}{2} \times 9 \times 12 = 54$ square inches. 4. **Calculate the perimeter of the triangle:** $$9 + 12 + 15 = 36 \text{ inches}$$ 5. **Calculate the lateral surface area:** $$\text{Perimeter} \times \text{height} = 36 \times 12 = 432 \text{ square inches}$$ 6. **Calculate total surface area:** $$2 \times 54 + 432 = 108 + 432 = 540 \text{ square inches}$$ 7. **Convert to square feet:** Since $1 \text{ ft} = 12 \text{ in}$, then $1 \text{ ft}^2 = 144 \text{ in}^2$. $$\frac{540}{144} = 3.75 \text{ square feet}$$ **Final answer:** The surface area of the triangular prism is $3.75$ square feet.