If the height of an octagonal prism is 11, what is the volume of the box?
Well, if the base has area B, then the volume is 11B
So, I multiply the L, H, and W of 11 and then add it all up?
yes.
That indicates you're dealing with a rectangular prism, not an octagonal one. The prism does in fact have 8 vertices, but prisms are usually referred to by the shape of their base. In this case, that's a rectangle, not an octagon.