Question 157554
I'm assuming that you mean a "regular" polygon!
The measure of an interior angle, A, of a regular polygon of N sides is given by:
{{{A = (N-2)180/N}}} Substitute A = 120 degrees.
{{{120 = (N-2)180/N}}} Multiply both sides by N.
{{{120N = 180N-360}}} Subtract 120N from both sides.
{{{0 = 60N-360}}} Add 360 to both sides.
{{{360 = 60N}}} Finally, divide both sides by 60.
{{{N = 6}}} 
The regular polygon has 6 equal sides and is called a hexagon.