a) How much NaHCO2 (sodium formate) would you have to add to 0.10 M HCO2H (formic acid) to get a buffer solution with a pH of 3.1? (HCO2H: Ka = 2 x 10-4)? b) what would be the pH after 10 mL of 0.1 M sodium hydroxide was added to 100 mL of the buffer prepared in part a?

This is all I came up with...

ka=[nh4c2h3O2]\[nh4oh][ch3cooh]