Okay, I see whats happening here.
It is asking for the volume. Now, you can see how it's split into three shapes right?
Two triangular prisms, and one rectangular prism.
What you need to do is find the volume of all three shapes and add them together.
You simply need to do what you did with question 1 and 2, and put them to use in number 3.
Remember: The two triangular prisms are equal in all sides, so you only have to calculate one of them.
Hint: Because the whole length of the figure is 12 cm, and the rectangular prism's length is 6 cm, this means that the base of the triangular prisms is 2 cm and 3 cm.
I hope this cleared up how to do this.