Ionic Equilibrum

A buffer solution is prepared by mixing $0.2 \mathrm{~M} \mathrm{~NH} \mathrm{O}_4 \mathrm{OH}$ and $1 \mathrm{~M} \mathrm{~NH}_4 \mathrm{Cl}$.
What is the pH value of buffer solution?
$\left(\right.$ Give $\left.\mathrm{pK}_{\mathrm{b}}=7.744\right)$