What's a regular heptagon? "Hept-" means seven, so a "heptagon" is a seven-sided shape. A "regular" figure's sides are all the same length. So, each of the 7 sides are the same length.
Remember, perimeter is the sum of the lengths of all the sides in a polygon. We know one side measures three inches, so each of the others must also measure 3 inches. 3+3+3+3+3+3+3=3*7=21 inches.
Answer: 21 inches