Hi, I have a Biochem question regarding buffers.

so the problem is, describe how you would prepare a 1L of 0.20 M lactate buffer with a pH of 4.2. what ratio of lactate sale to lactic acid would you use? the pKa of lactic acid is 3.86.

I set up the Henderson hasselbach equation and got to this point…

[A-] = (2.19)[HA]

in the solutions it goes on to say the equation is plugged into another equation and becomes..

(2.19)[HA] + [HA] = .20 M

I have no idea why it becomes…

(3.19) + [HA] = .20 M

if you can explain it me, I would appreciate it! thank you!