In an employment discrimination action, the Ninth Circuit's reversal of the district court's dismissal of the action based on the parties' agreement to arbitrate is reversed where, under the Federal Arbitration Act, where an agreement to arbitrate includes an agreement that the arbitrator will determine the enforceability of the agreement, if a party challenges specifically the enforceability of that particular agreement, the district court considers the challenge, but if a party challenges the enforceability of the agreement as a whole, the challenge is for the arbitrator.