Can someone please explain how to do this question?

Question

A 0.057M solution of a weak acid has a pH of 4.93. Determine the ionization constant, Ka, of the acid.
a) 2.1 x 10^-4
b) 2.4 x 10^-9
c) 4.1 x 10^-8
d) 2.8 x 10^-7
e) 1.4 x 10^-10
-- Well, to be honest, I skipped this. I know that the Ka = ((H3O^+)(A^-))/(HA), but I didn't know how to find (A^-). Am I approaching this question wrong? Is there a step I missed or can this be solved another way? Perhaps it has something to do with an ICE chart?

Thank you for your time, I really do appreciate your help.