Cu + Cl2 ---> CuCl there has to be the same amount of each element on either side. You have 1 Cu on either side, but you have 2 Cl on the left, and only one on the right. That means that you have to multiply CuCl by 2 in order to get the right number of Cl on each side. The problem is that you then have an extra Cu on the right, so you have to multiply the Cu on the left so that it is balanced. You wind up getting 2Cu +Cl2----> 2CuCl Hope this helps! Make sure to try some similar problems on your own so that you make sure you understand the concept.
