n = number of sides
R = angle of rotation
It turns out that
R = 360/n
since there are 360 degrees in a full rotation, which is then split up equally into n
different pieces
For example, a regular quadrilateral (aka "square") has n = 4 so
R = 360/n = 360/4 = 90
meaning we an rotate a square 90 degrees to have it line up with itself. Every 90 degree rotation, this will happen.
Note: This trick only works with regular polygons