What is the pH of a solution if the [H3O+] is 6.7*10-4m?

You need 5 basic conversions (some give you a 6th, but all you need are these 5):

1) pH = - log [H+] (negative log of the concentration of the hydronium ion) thus,

2) [H+] = 10 ^ - pH.

3) pOH = - log [OH-] and thus,

4) [OH-] = 10 ^ - pOH

5) pH + pOH = 14