-decay: . Therefore and .
You're told the KE of the alpha particle. Convert this energy from MeV into J (that is, convert to SI units). Now use the formula (that you should know) that links KE and velocity and solve for v (the unit will be m/s) ....
There's a mass difference between the mass of the original nucleus and the masses of the helium-4 (the alpha particle) and Fermium-252. And you know