The another side is the 11 cm .
reason for the answer:
We have one property that is
(1) if the sum of the any to sides of the triangle should be greater than the third side.
(2) if the difference of any two sides of the triangle should be lesser then the third side .
By this we check that in the triangle you already said that is the isosceles triangle so it should be 5 cm (or ) 11 cm.
By the above property if you check that
then the answer should be 11 cm.
so, answer is 11 cm.