apply Nagell-Lutz theorem: first of all the (non-zero) torsion points have integer coordinates. if has order 2, then and therefore which has no non-zero solution in

so, there is no point of order 2 on the curve. if has order at least 3, then which gives us using these possible values of find possible values of by solving

finally accept only those which have finite order. (remember the theorem only gives a necessary condition!)