(3-17)  We have (equation III-8): 

①  First square both sides, then multiply both sides by the denominator from the right hand side.

②  Separate out the terms on the left, to get an expression of the form E2 = ……  . Then substitute the original expression for E back in on the right hand side.

③   Now for the clever bit. Recall the expression we derived for relativistic momentum (equation III-3):      

Use this to simplify the right hand side of your expression for E2 .