What Is the [H₃O⁺] in a Solution That Consists of 1.5

What is the [H₃O⁺] in a solution that consists of 1.5 M NH₃ and 2.5 M NH₄Cl? K_b = 1.8 × 10^-5

A) 1.1 × 10^{-5 M}
B) 3.0 × 10^{-6 M}
C) 3.3 × 10^{-9 M}
D) 5.6 × 10^{-10 M}
E) 9.3 × 10^{-10 M}