We can get the Ka value from this formula:
Ka = [H+] [A-} / [HA]
So we need to know [H+] & [A-] & [Ha] to get the Ka value
[H+] = 2.9 * 0.52/100 =0.01508 M
and when [H+] = [A-]
∴[A-] = 0.01508 M
∴ [HA] = 2.9 - 0.01508 = 2.88 M
So by substitution:
Ka = [0.01508][0.01508] / 2.88
= 7.9 x 10^-5
