No rule for this one
No ending or word group predicts the article of Schrank. Learn it as a pair and say it out loud: der Schrank.
der Schrank
It's der Schrank. Schrank is masculine, plural die Schränke.
Der Schrank ist groß.
The wardrobe is big.
No ending or word group predicts the article of Schrank. Learn it as a pair and say it out loud: der Schrank.