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 Homework Statement

Two balls of mass 4kg and 2kg are moving with speed 10 m/s and 8 m/s, respectively with ball of heavier mass behind the lighter ball. The two balls collide each other elastically. Answer the following questions:
a) The common velocity of the system of balls during collision will be
b) Maximum potential energy stored in the system of two balls during the collision will be about
P.S
Answers a) 9.3m/s, b) 2.6 J,( according to the book)
 Homework Equations
 Conservation of linear Momentum.
My attempt
a) used equation { MV + mv = (M+m) V' }
and got the right answer.
b) I assumed that potential energy was asked for when the two balls were moving together with velocity 9.3m/s, so
I used that when before the collision K.E( of m) + K.E( of M) will be equal to K.E(of M+m) +P.E( of M+m) or
K.E(m) + K.E(M) = K.E(M+m) +P.E(M+m)
But i got the wrong answer, i got P.E =4.53 , why this is wrong ?
a) used equation { MV + mv = (M+m) V' }
and got the right answer.
b) I assumed that potential energy was asked for when the two balls were moving together with velocity 9.3m/s, so
I used that when before the collision K.E( of m) + K.E( of M) will be equal to K.E(of M+m) +P.E( of M+m) or
K.E(m) + K.E(M) = K.E(M+m) +P.E(M+m)
But i got the wrong answer, i got P.E =4.53 , why this is wrong ?