\(\frac{- GMm }{ R }+\frac{1}{2} m \frac{ V _{ o }^{2}}{9}=-\frac{ GMm }{ R + h }\)
\(\frac{ GM }{ R + h }=\frac{ GM }{ R }-\frac{ V _{ e }^{2}}{18}\)
\(\frac{ GM }{ R + h }=\frac{ GM }{ R }-\frac{ GM }{9 R }\)
\(\frac{ GM }{ R + h }=\frac{8 GM }{9 R }\)
\(\frac{1}{ R + h }=\frac{8}{9 R }\)
\(9 R =8 R +8 h\)
\(h =\frac{ R }{8} \Rightarrow \frac{6400}{8} \Rightarrow 800 km\)