#### Answer

perimeter = 24 mm

#### Work Step by Step

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