pKa value calculation in acid-base titration

Hi all,

So I had a buffer and acid-base titration lab and need to write lab report.
In the lab, the buffer was made with 0.45g KH2PO4 and 1.15g K2HPO4 with 100ml water.
I got titration curve and derivative plot (for both titrant, HCl and NaOH) to identify the equivalency point.
Now, I need to get pKa value for buffer.
Can anyone help me how to find pKa value??

Also, after finding the pKa value for buffer, I need to find out molecular weight and ionic strength for unknown amino acid. Could you please tell me how are the steps like to figure out the unknown amino acid?
Thank you:)