I think that there is extraneous information in this question.
On a related and more complex matter, if the problem had asked for calculation of the concentrations of acetic acid (HA) and acetate ion (A1-), these could be obtained with the H-H equation, but one would use an apparent pKa at high ionic strength. At the stated concentration of 1.4 M, the ionic strength is quite a bit higher than for the thermodynamic pKa, which is zero. Biochemistry laboratory courses sometimes discuss this problem, but it may not be a concern for you. Others here would know more about this topic than I do, but it is a matter of some practical concern when preparing buffers.