imagine the theoretical monoprotic acid HA where H is hydrogen and A is a variable.
HA<=H20=>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!