This the chemical equation of the reaction of Al and HCl
Al + HCl → AlCl3 + H2 (g)
When you balance the atomos you get:
2Al + 6HCl → 2AlCl3 + 3H2 (g) ------- you can check that here you have the same number of atoms of Al, H, and Cl in both sides.
The coefficients are:
2 for Al, 6 for HCl, 2 for AlCl3 , and 3 for H2.
Then, the answer is: the coefficient of HCl is 6
