Find the final velocity of ball B if the collision is elastic

Ball A of mass 5.40-kg, moving to the right at a speed of 2.10 m/s on a frictionless table, collides head-on with ball B of mass 6.80-kg moving to the left at a speed of 2.20 m/s . Find the final velocity of ball B if the collision is elastic.Take right as positive and give your answer in SI units.

both momentum and energy are conserved

momentum
M1V1 + M2V2 = M1V3 + M2V4
5.40(2.10) + 6.80(-2.20) = 5.40V3 + 6.80V4
11.34 - 14.96 = 5.40V3 + 6.80V4
-3.62 = 5.40V3 + 6.80V4
5.40V3 = -6.80V4 - 3.62
V3 = -1.26V4 - 0.67

energy
1/2 M1 V1^2 + 1/2 M2 V2^2 = 1/2 M1 V3^2 + 1/2 M2 V4^2
cancel all the 1/2's
M1 V1^2 + M2 V2^2 = M1 V3^2 + M2 V4^2
5.40(2.10)^2 + 6.80(-2.20)^2 = 5.40 V3^2 + 6.80V4^2
56.726 = 5.40 V3^2 + 6.80V4^2
substitute from momentum result
56.726 = 5.40 (-1.26V4 - 0.67)^2 + 6.80V4^2
56.726 = 5.40(1.59V4^2 + 1.69V4 + 0.45) + 6.80V4^2
56.726 = 8.59V4^2 + 9.13V4 + 2.43 + 6.80V4^2
56.726 = 15.39V4^2 + 9.13V4 + 2.43
15.39V4^2 + 9.13V4 - 54.3 = 0
V4 = 1.60 or 2.20
but only 2.20 m/s satisfies the conservation of momentum

I am troubled by the results but the concepts are correct
so
check the math

kt - I made an error at the end. 1.6 is the correct answer.
The problem has 2 unknowns in it - V3 and V4.
You need 2 equations to solve for 2 unknowns.
So I wrote the cons of mom to get V3 in terms of V4.
Then I wrote the cons of en and subbed to get V4.
I put the roots back into mom to pick.

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