What is the pH of a solution in which [H3O^+] = 6.8 x 10^-5 M?

What is the pH of a solution in which [OH^-] = 2.5 x 10^-9 M?

1.

pH = -log(H^+)

2.
(H^+)(OH^-) = Kw = 1E-14.
You know Kw and OH, solve for H^+, then use pH equation in #1.