What is the value of [OH-] in a 0.015 M CH3COOH solution? Ka = 1.8 x 10-5

Question

I started by writing the equation:

CH3COOH + H2O <---> H3O+ + CH3COO-

Ka= [H3O+][CH3COO-]/[CH3COOH]= 1.8 E-5

... Now what? I don't understand where OH relates to this ...