Answer:
18cm^2
Step-by-step explanation:
First, work out all the common factors, then take the highest.
90: 1 , 2 , 3 , 5 , 6 , 9 , 10 , 15 , 18 , 30 , 45 , 90
126: 1 , 2 , 3 , 6 , 7 , 9 , 18 (+ (there's no more that are the same as that of 90))
So, the highest common factor is 18, therefore, 18 is the longest side you can have