$NH_3 + O_2 \to NO + H_2O$
$\left.\begin{array}{cc}4 \times 17\,g & 5\,mol \\ 6.8\,g & (?)\end{array}\right\}=\frac{6.8 \times 5}{68}=0.5\,mol$
$^{\mathrm{M}}\left[\mathrm{Co}\left(\mathrm{NH}_{3}\right)_{6}\right] \mathrm{Cl}_{3}=267.46 \;\mathrm{g} / \mathrm{mol}$ $\mathrm{M}_{\mathrm{AgNO}_{3}}=169.87 \;\mathrm{g} / \mathrm{mol}$