Answer is:
concentration of hydroxide ions is 2.38·10⁻¹⁰ M.
[H₃O⁺] = 4.2·10⁻⁵ M; equilibrium concentration of hydroxide ions.
The Kw (the ionic product for water) at 25°C is 1·10⁻¹⁴ mol²/dm⁶ or 1·10⁻¹⁴ M².
Kw = [H₃O⁺] · [OH⁻].
[OH⁻] = Kw ÷ [H₃O⁺].
[OH⁻] = 1·10⁻¹⁴ M² ÷ 4.2·10⁻⁵ M.
[OH⁻] = 2.38·10⁻¹⁰ M.