induction a.
simpl.
trivial.
replace (S a*n) with (n+a*n);try (auto with arith).
rewrite plus_comm.
rewrite plus_minus_lemma1.
simpl.
rewrite <- minus_n_O;trivial.
Qed.

(* Specifications: *)
Lemma mult_lemma6 : forall (a b n:nat),(n <> O)->(n*a=n*b)->(a=b).
induction a.