It is: 180(n-2)
For example, the sum of the interior angles of a triangle is 180 degrees.
The quadrilateral can be cut into two triangle so the sum of the interior angles of a quadrilateral is 180*2=180*(4-2)
The pentagon can be cut into three triangle so the sum of the interior angles of a pentagon is: 180*3=180*(5-2)
The hexagon can be cut into four triangle so the sum of the interior angles of a hexagon is: 180*4=180*(6-2)
