Solving a chemistry problem

4. To prepare the buffer system, V1 ml of a solution of a weak acid (weak base) with a molar concentration of the equivalent of c1 and V2 ml of a salt solution with a molar concentration of the equivalent of c2 were mixed. V3 ml of strong acid with concentration c3 was added to the buffer solution. Calculate:
a) pH of the buffer solution (initial pH value), dissociation constant of a weak acid, coefficients (or weak base) are given in Table 13.
b) pH of the buffer solution after adding a strong acid
c) buffer capacity of the buffer solution
d) buffer zone;
e) explain the mechanism of the buffering action when a strong acid is added to a buffer solution.
Table N13
N acid salt c1,
mol/l c2,
mol/l c3,
mol/l V1, ml V2, ml V3, ml K¬d
11-4 H2CO3 NaHCO3 0.1 0.1 0.15 5 5 2 4.69*10-7