if a week acid has a dissociation constant of 6.4x10-6 what is the pH of a solution of this acid in 0.10 molar concentration?

I think there are a lot of similar questions already answered on AskMe.
Using the acid HA, set up the equation for K, let the concentration of H+ and A- be X, and let the concentration of acid stay at 10^-1M (the dissociation does not make a significant change in concentration). Solve for H+, use the definition for pH.
I wonder if you looked for WEAK acids in your textbook you would find an explanation for this problem.