The demo project shows how to draw a circle using Pen blocks. Make sure you understand how that works. The main thing you need to be aware of is that a full rotation is accomplished when the number of repeats multiplied by the angle within the repeat block equals 360 degrees. To make a rainbow you will be creating arcs or half circles.

View project at Scratch

There are seven colors in a standard rainbow and they are red, orange, yellow, green, blue, indigo, and violet. ROYGBIV is an acronym used to help people remember these colors. A couple mnemonics exist to help recall these colors, but you can look those up yourself if you are sufficiently interested. At any rate, getting back to the assignment, your job is to create a rainbow using Pen blocks. The left side of your rainbow should be at (-140,0) and the right side of your rainbow should be at (140,0). There should be no gaps between the seven colors of the rainbow.