3.00×10−3 mol of HBr are dissolved in water to make 16.0 L of solution.What is the concentration of hydroxide ions, [OH−] in this solution?

Concentration of HBr = (3.00 × 10⁻³ mol) / (16 L) = 1.875 × 10⁻⁴ M

HBr is a strong acid which completely dissociates to give H⁺ ions.
Hence, [H⁺] = 1.875 × 10⁻⁴ M

[OH⁻] = Kw / [H⁺] = (1.00 × 10⁻¹⁴) / (1.875 × 10⁻⁴) M = 5.33 × 10⁻¹¹ M

Just for a variety of methods:

pH = - log ((3.00 × 10^−3 mol) / (16.0 L)) = 3.727
pOH = 14 - pH = 10.273
10^-10.273 = 5.33 × 10^-11 M OH-