How Is the Mass of the Target Nucleus Calculated in an Elastic Collision?

[Emendez3]

A proton (mass 1 u) is shot toward an unknown target nucleus at a speed of 2.40×10^6 m/s. The proton rebounds with its speed reduced by 25% while the target nucleus acquires a speed of 3.50×10^5 m/s.

What is the mass, in atomic mass units, of the target nucleus?




I tried:
Mp*Vip+Mn*Vin=Mp*Vfp+Mn*Vfn

where Mp= Mass proton=1
Vip= intial proton velocity=2.4E6
Mn= Mass nucleus=unknown
Vin=Initial Velocity of nucleus=0
Vfp= Final velocity proton=.75(2.4E6)
Vfn=final velocity nucleus= 3.5E5


i get 1.71u but its wrong

 

[borgwal]

The final proton velocity has a minus sign

 

[Emendez3]

The final proton velocity has a minus sign

Thank you very much got the answer