I’m studying for my biochemistry final and encountered a pH problem that I don’t know how to approach.

A compound is known to have a free amino group with a pKa of 8.8, and one other ionizable group with a pKa between 5 and 7. To 100 mL of a 0.2 M solution of this compound at pH 8.2 was added 40 mL of a solution of 0.2 M hydrochloric acid. The pH changed to 6.2 . Find the pKa of the second ionizable group.

The answer is 5.6 but I don’t know how to calculate it.

Thanks for your help