First find the total BTU's by multiplying 750 by 15.
750 * 15 = 11,250 Total BTU's.
Next find the volume of the bus by multiplying the length by the width by the height.
Volume = 38 x 8.5 x 6.25 = 2,018.75 cubic feet.
Final step, divide the Total BTU's by the Volume of the bus.
11,250 / 2,018.75 = 5.57 BTU's per cubic foot.