Height = h = 850 km = $850\times 1000$ m = $0.85 \times 10^{6}$ m

Orbital velocity = $v_{o}$=?$v_{o}=\frac{√(〖 GM〗_{e}}{(R+h))}$

 

$v_{o}$ =$\frac{√((6.673\times 10^{-11}\times 6.0\times 10^{24})}{(6.4\times 10^{6}+0.85 \times 10^{6}))}$

=$\frac{√((40.038 \times 10^{13})}{(7.25 \times 10^{6}))}$

      = $√(5.55 \times 10^{7})$ = $√(5.55 \times 10^{6}$)

      = $7.431\times 10^{3}$ = $7431 ms^{-1}$