Please help! On the verge of tears :') (see pic)

Volume is l x w x h.
The bottom part is 3 x 10 x 10. The third dimension of it isn’t 12 because we’re splitting that up into the other shape. It’s 12 - 2 because of the small part we take away. 3 x 10 x 10 = 300 in
The second part is 2 x 10 x 5. The third dimension of it isn’t 8 because we’re taking part of it away due to the other shape, so 8 - 3 = 5. 2 x 10 x 5 = 100 in
So I believe it is 300 + 100 = 400 cubic inches
Sorry it’s hard to explain

Where’s the