2 smooth spheres of masses m and 3m move with speeds u and v in opposite directions . the smaller mass is brought to rest .i show u greater than or equal to 3v

im trying to working this out using the 2 equations m(u)+3m(-v)=m(0)+3m(x) where x is the final velocity of the 3m mass . im also using 0-q/u+v =-e