What is the perimeter of a regular heptagon if one side measures 3 inches

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.

21 inches

Are you trying to figure out the volume?

The decimal expansion would be .27 repeated