If enough of a monoprotic acid is dissolved in water to produce a 0.0150 M solution with a pH of 6.56, what is the equilibrium constant, Ka, for the

If enough of a monoprotic acid is dissolved in water to produce a 0.0150 M solution with a pH of 6.56, what is the equilibrium constant, Ka, for the acid? I got 5.04E10 to -12 but it says its wrong and i havent accounted for the autoprotolysis of water