Explanation:
You can imagine this figure as a rectangle and cube
If you want volume of this irregular figure than you have to do it like this:
V(figure)= V(rectangle)+ V(cube)
V(figure)= a*b*c+ a³
V(figure)= 4*3*(I don't see dimension on the left)+ 3³
V(figure)=12*(I don't see dimension on the left)+ 27
And only you have to to do is to set this dimension which I can't see.