Answer:
24 cm
Explanation:
We have the following: 
shortest side: 10 cm 
longest side: x 
diagonal: x + 2 
we can calculate the diagonal by means of pythagoras and it would be: 
(x + 2) ^ 2 = 10 ^ 2 + x ^ 2 
We solve this and we are left with: 
x ^ 2 + 4 * x + 2 ^ 2 = 100 + x ^ 2 
4 * x +4 = 100 
x = (100-4) / 4 
x = 24 
which means that the longest side is 24 cm.