please help with 1 a and b

1. Answer the following question: (a) Estimate carbon dioxide content of a natural water sample having a pH of 5.5 and bicarbonate ion concentration (HCO3-) of 28 mg/L. Assume temperature is 25C and KA1 = 4.3 x 10-7. (b) If the CO2 content was reduced to 3 mg/L by aeration, assuming bicarbonate ion is still the same, calculate pH of the above sample