Calculating OH- in a solution

Question

Calculate the [OH^-] in a solution in which the [H3O⁺] = 4.2 x 10^-9 M

Answer 1

[H₃O⁺][OH^-] = 1.0 x 10^-14 M²

[OH^-] = 1.0 x 10^-14 M²/[H₃O⁺]

[OH^-] = (1.0 x 10^-14 M²)/(4.2 x 10^-9 M)

[OH^-] = 2.38 x 10^-6 M