The volume of the prism is
V = Bh
where B is the area of the base, (5 ft)² = 25 ft², and h is the height, 15 ft.
The volume of the pyramid is
V = (1/3)Bh
where B is the area of the base, 25 ft², and h is the height, 3 ft.
The total volume is the sum of these:
V = Bh₁ + (1/3)bh₂
V = B(h₁ + (1/3)h₂)
V = (25 ft²)·(15 ft + (1/3)·(3 ft))
V = (25 ft²)·(16 ft) = 400 ft³