Answer
$6.5\times10^6 m$.
Work Step by Step
From Figure 1-16, we see that the distance d is a tangent line, and hence is perpendicular to the radius R. A right triangle is formed, with legs of R and d, and a hypotenuse of R + h. Use the Pythagorean Theorem.
$$d^2+R^2=(R+h)^2=R^2+2Rh+h^2$$
$$ d^2=2Rh+h^2$$
Because h is much less than R, we may neglect the term $h^2$ because it is so much smaller than the term 2Rh.
$$ d^2\approx 2Rh$$
$$ R=\frac{d^2}{2h}=\frac{(4400m)^2}{2(1.5m)} 6.5\times10^6 m $$
This is surprisingly close to the accepted value.
