Find the prime factorisation of following number using the successive division method: $616$

The prime factorisation of two numbers are:

$2\times 3\times 3\times 7\times 7\times 7\times 13$ and $3\times 7\times 7\times 13\times 13\times 13\times 17$.

Write down three common factors of the numbers.

A rectangular piece of paper measuring 35 cm by 28 cm is cut to obtain identical squares. Find the largest possible length of a side of each square.