[H+] = 10¯3.68 = 2.09 x 10¯4 M

[A¯] = [H+] = 2.09 x 10¯4 M

[HA] = 0.45 - 0.000209 = 0.45 (for all intents and purposes)

Ka = [(2.09 x 10¯4) (2.09 x 10¯4)] / 0.45

Ka = 9.7 x 10¯8