The highest point is 12cm, and the other part is 8, so the difference between the lower and higher prisms is 4.
So I'm not going to split this apart and instead I'm going to actually just calculate the total volume and subtract the part that's not filled.
So the "dug out" portion of the prism has a volume of 4 * 6 * 10 = 240. The total, including that, is 15*12 * 6 = 1080.
We counted the 240, so 1080-240 = 840
If you dont understand anything feel free to ask.