#### Answer

perimeter = $30 \text{ cm}$

#### Work Step by Step

RECALL:
(1) The perimeter of a polygon is equal to the sum of all the lengths of its sides.
(2) Opposite sides of a rectangle have equal lengths.
Using (2) above, the unknown side lengths are 9 cm and 6 cm.
Thus, the perimeter of the rectangle is:
$=(9+9+6+6) \text{ cm}
\\=30 \text{ cm}$