Calculating Escape Velocity for a Small Asteroid with Given Initial Velocity

[Berkshire]

Homework Statement

The escape speed from a very small asteroid is only 28 m/s. If you throw a rock away from the asteroid at a speed of 39 m/s, what will be its final speed?
Vesc= 28m/s
Vi= 39 m/s

Homework Equations

Ks,f + Ke,f + Uf = Ks,i + Ke,i + Ui +W
Vi=sqrt(2GM/r)

The Attempt at a Solution

Well I know the first equation gets simplfied to
0=.5mv^2 + (-GM/r)
Vi=sqrt(2GM/r)

But I don't have radius or mass of my asteroid and I don't have any equations that incorporate the final velocity...

But I can't figure out how to

 

[zhermes]

How can you relate the asteroid's escape velocity to the values you're looking for?