Solid Mensuration: truncated octagonal right prism

The top plane of the truncated octagonal right prism shown in the figure below is 45° with respect to horizontal. Find the volume.14636615518501494221760.jpg

What I did was to cut the figures into 2 separate figures, So that i can have a regular octagonal right prism. Then I computed for the regular's volume. Since the angle is 45 I assumed that the remaining figure is half the regular octagonal prism i separated. Therefore, the total volume= regular x 1.5