Sort data in descending order using Firebase

Ionic Framework ionic-v3
1

How do I sort user to orderBy user from the last contact added to the first. Descending order

voila mon code

export class Page1 {
persones:FirebaseListObservable;

constructor(public navCtrl: NavController,public alertCtrl : AlertController , public angFire:AngularFire) {
this.persones=angFire.database.list(’/Persones’);

}

les variable
register(nom , profession ,telefone ,ville , description)

help me plz