Let me see if I can help...
First let's split the shape into two rectangles.
Use the formula V = lwh or V = Bh ....
To find the volume of the bottom rectangle we multiply...
V = (4*2) x 1
This gets us 8 in³.
To find the volume of the top triangle we multiply...
V = (2*1) x 2
This gets us 4 in³.
Now just add 8 + 4 and you will get 12 in³.
Hope this helps you.
(If you need any more explanation I'm happy to help you.)