A assuming no volume change calculate the ph of the buffer

Question

A buffer solution was dissolved by adding 5.45 g sodium formate (Molecular weight of HCOONa is 68.008 g/mol) in 250 mL of 0.30 M formic acid (HCOOH) solution (K_a = 1.8×10^-3).

(a) Assuming no volume change, calculate the pH of the buffer solution using the Henderson-Hasselbalch equation.

(b) Calculate the pH of the above buffer solution after adding 15.0 mL of 1.0 M potassium hydroxide (KOH).