Hint:

S(3) = 180

S(4) = S(3) + 180

S(5) = S(4) + 180

. . .

S(n) = ???

Try making a polygon like the triangle below and then adding triangles to it to make a new polygon.

Make a table with number of sides and angle sum.

You should be able to tell from one polygon, what the angle sum will be for the next angle sum. Come up with a formula that expresses this relationship.