How Do I Solve the Momenergy Problem in Physics Homework?

[athrun200]

Homework Statement

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Homework Equations

The Attempt at a Solution

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I am not sure if I answer it correctly.
Can anyone help me to check?

 

[Spinnor]

E_k = Mv^2/2 = 2*5E6*42^2/2 = 8.8E9J = mc^2

solve for little m which will be in kg so you must convert to milligrams.