Formula for sum of interior angles is: 180(n-2). n is the number of sides the shape has. The shape with 8 sides, the octagon, has a sum of interior angles of 1080 degrees. Since it’s regular you would divide it by the number of angles it has (8) which gives you 135. In the regular octagon, each angle would be 135 degrees. Now you do the same for the 12 sided shape, the dodecagon. This would give you 150 degrees for each angle. 150-135 is 15. Therefore, the difference between the two angles is 15 degrees.