In order to solve we remember that the sum of internal angles of a polygon is given by:
[tex]\text{anlge}=(n-2)\cdot180[/tex]Where "n" is the number of sides.
So, we solve now:
[tex]3420=(n-2)\cdot180\Rightarrow n-2=19[/tex][tex]\Rightarrow n=21[/tex]So, the polygon has 21 sides.
