imagine the theoretical monoprotic acid HA where H is hydrogen and A is a variable.

HA<=H_{2}0=>H^{+} + A^{-}

Ka = ([H^{+}][A])/[HA]

use the ICE (initial change equilibrium) technique and probably the quadratic formula if the acid has enough strength to affect [HA] when dissociated, to find [H], [A], and [HA] in the solution when [HA]_{0} is 0.42M.

What i think you did was to skip the ICE step, you have to remember that a .42M solution of an acid means it is diluted, and that the H^{+} ions which dissociate will determine the pH.

have fun typing quadratic formula solutions into your calculator!