PH buffer concentration question plz explain and help tnx much :D?

Question

what is the concentration of a lactic acid buffer (pKa=3.9) that contains .25 M CH3CH(OH)COO-? what is the pH of this buffer?

- is the negetive sign on the COO :D

Answer 1

pH = pKa + log(A-/HA)

pH = 3.9 + log(0.15 / 0.25)

pH = 3.9 + (-0.22) = 3.7

Answer 2

pH = pKa + log [lactate]/ [lactic acid]

I kwon only pKa and [lactate]

I do not knw ph and [lactic acid]

I require more data