We are asked to find the concentration of a solution of HCl if the resulting proton concentration is 6 M. HCl is a strong acid. As such, it completely ionizes into H+ (protons) and Cl-. So every mole of protons in the solution were produced from a mole of HCl at the start. So if we have a solution that is 6 M in protons, that means we started with a solution that was 6 M in HCl to start with. So the answer is the HCl concentration was 6 M.