Hello, Sherpa!

Your formula is correct.

. . Your answers should have been correct.

Code:

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b

The triangular faces are all right triangles.

. . The area of a triangle is: .$\displaystyle \tfrac{1}{2}ab\text{ units}^2$

. . Hence, the area of *two* triangles is: .$\displaystyle ab\text{ units}^2$

There are three rectangluar panels.

. . Their areas are: .$\displaystyle ah,\;bh,\;ch\text{ units}^2$

Therefore, the total surace area is: .$\displaystyle S \;=\; ab + (a+b+c)h$