find the minimum coaltitude which can directly receive a signal from a geostationary satellite.

Explanation

The farthest point earth, which can receive signals from the paking orbit the point where a length is drawn on earth surface from satellite as shown in figure

The colatitude `lambda` of point P can be obtained from figure as
`sin lambda = (R_(e ))/(R_(e )+h)~=(1)/(7)`
we know for a parking orbit `h ~= 6R_(e )`
Thus we have `1 = sin^(-1)((1)/(7))`