Zn + 2HCl ==> ZnCl2 + H2 ... balanced equation
Looking at mole ratio of H2 : ZnCl2, we see it is 1:1
6 g H2 x 1 mol H2/2 g = 3 mole H2
So, 3 mol ZnCl2 would have to be produced if 3 mol (6 g) of H2 were produced.
Mass ZnCl2 = 3 mol ZnCl2 x 136 g/mol = 408 g ZnCl2 (not 51 g)
216 g HCl x 1 mol HCl/36.4 g x 1 mol H2/2 mol HCl = 3 mole H2 formed as stated in the problem. This is the case since HCl is limiting.