Chapter 10 - Geometry - 10.3 Polygons, Perimeter, and Tessellations - Exercise Set 10.3 - Page 636: 20

perimeter = 24 mm

RECALL: (1) The perimeter of a polygon is equal to the sum of the lengths of all its sides. (2) The sides of a regular polygon have equal lengths. This means that the five unknown side lengths of the regular hexagon are all $4$ mm. Add the lengths of all the sides to find the perimeter: perimeter = $4+4+4+4+4+4=24$ mm