since it is a rectangle, the parallel sides will be congruent. one side will be the same as the opposing side, so the opposing side of the 3cm side will be 3cm, so you add those together, which is 6cm. The opposing side of the 8cm side will be 8cm as well, so you add those together, making 16cm. then you add your two totals and yay! you have your answer! Perimeter=22cm