Answer:
21
Explanation:
A = s^2

You need to find the square root of the area which is the square root of 441.
What number do you multiply by itself and get 441?
Try 15: 15^2 = 15 * 15 = 225 too low
Try 18: 18^2 = 18 * 18 = 324 still too low
Try 20: 20^2 = 20 * 20 = 400 still low but getting closer
Try 21: 21^2 = 21 * 21 = 441 this is it
s = 21 cm