how do you balance

Question

Al +Cl2-->Al+3 +Cl-

2 is a little subscript in the corner in the bottom.

+3 and - go on the top corner.

my teacher got
2Al + 3Cl2 --> 2Al+3 + 6Cl-

where does the 2 and the three come from

Answer 1

aluminum forms a +3 ion, and chlorine forms a -1 ion. So for every +3 ion formed, one needs three -1 ions.
So for every two +3 ions, one needs six -1 ions.
Because chlorine was diatomic in the original equation on the left, one needs to balance with six + charges and six -1 charges. The real idea here is to balance total charge, and total mass.