Since we have both acetic acid and sodium acetate in the solution it means we have a buffer. A buffer is a solution of both a weak acid and its conjugate base. When these 2 are present in fair amounts the solution can resist pH changes.
The solution reaction therefore is:
HA + H2O --> A- + H3O+ (this is before adding any HCl), (HA would be the acetic acid and A- resembles the sodium acetate)
When HCl is added to the solution it will react with the acetate ion since ...
Full solution with chemical equation for solution reaction and explanations. 350 words.