We separate the figure into two parts to be able to calculate each volume and at the end add them
triangular prism
Volume is area of the triangle by the length

where b is the base of the triangle, h the height of the triangle and l the the depth of the prism
replacing

the volume is 10,000 cubic feet
Rectangular prism
Volume is the area of the rectangle by the length

replacing

the volume of the rectangular prism is 30.000 cubic feet
Total Volume
we add the volume of both figures

The total Volume is 40,000cubic feet