Rockets 'n' Shit

A rocket is fired vertically from the earth's surface. The fuel is completely used up when the rocket reaches an altitude of 40 km, at which time the velocity of the rocket is 6500 ms^-1. What altitude will the rocket reach before being drawn back to earth due to the effect of gravity?

Note: Newton's Law of Gravitation states that the magnitude of the force between two bodies of respective mass m and M is

GmM/d^2

where d = the distance between the centres of the masses
      G = 6.673 * 10^-11 m^3 kg^-1 s^-2

F = ma = GmM/x^2
a = GM/x^2

v = INTEGRATE(GM/x^2) = -GM/x + C
v(x=40) = 6500 ; solve for C

v(x) = 0 ; solve for x