Calculate the [OH^-] in an Aqueous Solution That Is 0

Calculate the [OH^-] in an aqueous solution that is 0.125 M NH₃ and 0.125 M NH₄Cl.The value of K_b for NH₃ is 1.8 * 10^-5.

A) 1.8 * 10^-5 M
B) 5.5 * 10^-10 M
C) 6.7 * 10^-12 M
D) 0.125 M
E) 1.5 *10^-3 M