Strange stereometry

Question

A cube-shaped vessel with a 6 cm rib filled with water was turned through an angle of 90 degrees relative to one of the sides of the base. Find how much water remained outside the vessel.

I literally don't know what to do. I tried to calculate volume via proections,but with no success. I need detailed explanation.

Answer 1

The volume of the remaining water forms a prism with a right triangular base.

Answer 2

Hint

You can use a planar figure as with a cube, you are just sweeping the planar figure along the rotated edge.

Then the water on the planar figure is representing a rectangle triangle for which the area is not difficult to be computed.