The volume of the building would be the sum of the volume of the roof and the volume of the base
For the volume of the base, the formula is
V = lwh
Where
l = length of base = 50 feet
w = width of base = 15 feet
h = height of base = 15 feet
Volume of base = 50 * 15 * 15 = 11250 square feet
For the volume of the roof, the formula is
V = 1/2lwh
Where
l = length of roof = 50 feet
h = height of roof = 10 feet
w = width of roof = 15 feet
Volume of roof = 1/2 * 50 * 10 * 15 = 3750
Volume of buiding = 11250 + 3750 = 150000 cubic feet