What is the [OH-] in a solution that has [H3O+] = 2.0*10^-4M

I would really love an explanation on how to get the answer. Thanks is advance!

Find the pH first.

pH = -log[H3O+]

pH = -log[2.0 * 10^-4] = 3.70

Then, using this relation pH + pOH = 14, find the pOH

3.70 + pOH = 14

pOH = 10.3

Then find the OH concentration by working backwards using this pOH = -log[OH]

10.3 = -log[OH]

[OH] = 5.01 * 10^-11 M