No rule for this one
No ending or word group predicts the article of Mantel. Learn it as a pair and say it out loud: der Mantel.
der Mantel
It's der Mantel. Mantel is masculine, plural die Mäntel.
Der Mantel ist teuer.
The coat is expensive.
No ending or word group predicts the article of Mantel. Learn it as a pair and say it out loud: der Mantel.