Determine the resulting ph if 0.02 mol of hcl is added

Question

A buffer was consisted of 500 mL of 0.90 MNH₃ and 500 mL of 0.90 M NH₄⁺. The Ka of NH₄⁺ is 5.6 x 10^-10.

Determine the resulting pH if 0.02 mol of HCl is added to the original buffer and the resulting pH if 0.02 of NaOH is added to the original buffer.