Question

How many sides does a polygon with an interior angle measure of 120 degress have?

Answer 1

Check this informative site.

http://www.mathsisfun.com/geometry/interior-angles-polygons.html

Answer 2

sum of interior angles of a polygon is 180(n-2) where n is the number of sides

so solve

180(n-2)/n = 120

you should have stated that it must be a "regular" hexagon.