How to find a missing side length of a square?

The top and bottom length are 9 and 9 right? so multiply 9x2 which is 18cm. then to find the missing length, you already know that the length to your right is 6cm. and so will the one on your left will be 6cm, so multiply 6x2 and you get 12cm. add 18cm and 12cm and you get the answer 30cm. perimeter=30cm.

-3

Step-by-step explanation:

42/6=7

7-8=-1

-1x3=-3