How do you find the volume of a composite solid

To find the volume of a composite solid of a triangular prism on a cuboid first work out the area of the cross-section/ front face. The area of the square is the base of 10cm x the height of 12cm = 120, and because the triangle only has a 40-degree angle, cut it down the middle to give a 90-degree angle, and half the base is 10/2 = 5cm so because 40 is the opposite and 5 is the adjacent use Tan(o/a), then multiply by ½ x 10 x ½ area of the triangle to get the total area, and add this to the square. Now work out the volume = area of cross-section x depth(length) in cm^3