First, find the volume of the pyramid on top:
(2.5 * 3 * 1.5) / 3 = 3.25
Then, find the volume of the rectangular prism:
2.5 * 3 * 2 = 15
Add the two volumes together:
3.25 + 15 = 18.25
To explain: you found the volume of each separate part and added it together.