[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)

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

K_a = 9.7 x 10¯⁸