Science Homework Help

I did an online titration with an unknown acid and NaOH as the titrant. Please look at the photos linked. The first is prior to the titration, and the second is of the completed curve.

pKa = 5.60, therefore the Ka = 2.51×10^-6

volume of unknown acid solution = 25mL

concentration of NaOH used in the titration = 0.5M

I believe that the pH prior to starting the titration was 2.87, as you can see from the bottom left table on the first image.

Please show me how to find the concentration of the unknown acid. Thank you.