The answer is 0. If you plug it into a calculator it takes only 5 seconds.
0+12, 11+1, 4x3, 6x2, etc.
3420°
explanation:
the equation of the sum of interior angles are (n-2)x180
polygon above has 21 sides so you sub n as 21 and plug the equation in the calculator
It should be D Vietnam I believe and by the way you have this under mathematics.