When a polygon is regular, that means that all of its sides are the same length and all of its angles are the same. A heptagon has seven sides and you know that all of them measure 3 inches. You can find the perimeter by adding all of the side measures together, so 3 + 3 + 3 + 3 + 3 + 3 + 3, or 3 x 7. So, the perimeter of the heptagon is 21 inches.