You know the endpoints of one side of the pentagon, so you can calculate the length of that side. Use the distance formula: distance = sqrt((x2 - x1)2 + (y2 - y1)2). (You can use this formula to find each side of the square in the other problem that you posted, too.) Because the pentagon is regular, all five sides are equal in length, so the perimeter will simply be five times the length of this side. Let me know if you have any trouble when plugging in values.