Does anyone know the answer to the following question?

A solution is prepared from 2.00 mL of a 0.56 M solution of monoprotic weak acid and 2.00 mL of a 0.45 M of the conjugate base of the same acid, and the mixture is diluted to a volume of 5.00 mL with water. If the pH of the resulting solution is 5.15, what is the pKa of the monoprotic weak acid?