Once the boosters detach they are following an projectile motion; then we can use the following equation to determine its initial velocity;
Now, in this case the initial position is 47549 meters, the final position is 72542 meters, the acceleration is that of the gravity and the final velocity is zero (it has to be zero, otherwise the object will continue going upwards or it will be already falling). Plugging the values we know and solving for the initial velocity we have:
Therefore the initial velocity is 699.9 m/s