induction n1; destruct n2; intuition; simpl in *.
remember (n2 * n3) as x.
omega.
remember (n1 * n3) as x.
omega.
f_equal.
eapply IHn1; eauto.
eapply plus_reg_l. eauto.
Qed.

Lemma mult_same_l : forall n3 n1 n2,
n3 > 0 ->
n3 * n1 = n3 * n2 ->
n1 = n2.