This is made up of 4 triangular segments. If we assume the cross section is perpendicular then we can use the 1/2b*h formula for each piece with the given values. So we have 2 triangles with the formula: 1/2(3)*(4.5)=6.75 * 2 triangles = 13.5.
Then for the bottom 2 you have 1/2(3)*7.5=11.25, times 2 triangles =22.5
