$$\begin{gathered} {\mathrm{Ag}}^{+}+{\mathrm{NH}}_3\rightleftharpoons \mathrm{Ag}{\left({\mathrm{NH}}_3\right)}^{+};{\mathrm{K}}_1=3.5\times {10}^{-3} \\ \mathrm{Ag}{\left({\mathrm{NH}}_3\right)}^{+}+{\mathrm{NH}}_3\rightleftharpoons {\left[\mathrm{Ag}{\left({\mathrm{NH}}_3\right)}_2\right]}^{+};{\mathrm{K}}_2=1.7\times {10}^{-3} \end{gathered}$$
Then, the formation constant of [Ag(NH₃)₂]⁺ from Ag⁺ and NH₃ is