What is the pH of the solution created by combining 12.40 mL of the 0.10 M NaOH(aq) with 8.00 mL of the 0.10 M HCl(aq)?

with 8.00 mL of the 0.10 M HC2H3O2(aq)?

I know how to solve for HCl but not with HC2H3O2.

NaOH + HC2H3O2 ==> NaC2H3O2 + H2O

moles NaOH = M x L = ??
moles HC2H3O2 = M x L = ??
See which is in excess when NaOH is added to HC2H3O2. If NaOH is in excess the pH is determined by excess OH. If HC2H3O2 is in excess, then you have a mixture of acetic acid and sodium acetate which is a buffer and you use the Henderson-Hasselbalch equation.