The goal in this assignment is to work no more that 3 hours and draw like your in a race against time. Remember you are drawing cubes, so they have to look like cubes. Don't focus on getting it right, focus on it looking right, there is a difference. You want the perception of a cube not an exact cube. The reason for this is to develop the ability to create the illusion of reality, fast. Speed is god in professional art. You are pre visualize ideas quickly, this skill is essential to the development of ideas. If it isn't right, think of what you did wrong , make the change and try again.
Drawing so you can see through the cubes is recommended. You a starting to train your brain to see in 3D. More is best.