Use the segment tool to [i]triangulate[/i] each polygon: Pick [u]ONE[/u] vertex and draw all of the possible diagonals from that [u]ONE[/u] vertex. This will divide the polygon into multiple triangles.
Compare the number of triangles created in each polygon to the number of sides in the polygon. What pattern do you notice?
We know that there are 180˚ in every triangle. How can you use this to help you figure out how many total degrees are inside each polygon (interior angle sum)?
What is the sum of the interior angles of a pentagon?