The $\mathrm{OH}^{-}$concentration in a mixture of 5.0 mL of $0.0504 \mathrm{M} \mathrm{NH}_4 \mathrm{Cl}$ and 2 mL of $0.0210 \mathrm{MNH}_3$ solution is $\mathrm{x} \times 10^{-6} \mathrm{M}$. The value of x is $\_\_\_\_$ . (Nearest integer) [Given $\mathrm{K}_w=1 \times 10^{-14}$ and $\mathrm{Kb}=1.8 \times 10^{-5}$ ]