Apiece for a board game is shaped like a triangular prism. the piece is 18 millimeters long. the base of the piece is an equilateral triangle with 10 millimeter sides and a height of 9 millimeters. find the surface area of the game piece

Area of the triangle, 
                       A = bh / 2 = (10 mm)(9 mm) / 2 = 45 mm²
Since there are 2 triangles, their total area is 90 mm².

Then, the area of the sides are,
            3A = (3)(10 mm)(18 mm) = 540 mm²
The total area is equal to 585 mm².