(5.a) 1.8 x 10^-5

This is the [H⁺] due to the HAc dissociation when NOT in the presence of 0.10 M HCl. This does not take into account the fact that the HCl has completely dissociated and this large concentration of H⁺ affects the dissociation of the HAc. This is an example of a "common ion" effect.