So at this point, we have a full rotation of 360 degrees round that point. The graph accounts for 36 degrees of that 360 degrees leaving 360-36=324 degrees left to be split between the three polygons at (Around) that point. So 324/3=108. Now we got to figure out a polygon that has all of it's angles being 108 degrees.
We can usually find that if we know the number of sides of the polygon but we don't but here is the formula 180(n-2)/n=108