Draw a bar model: total = 3 equal units. Shade 2 units lost, 1 unit left = given value. Multiply to find the whole.

Rex lost 2/3 of his cards, so the 12 cards left represent 1/3 of the original amount.
Draw a bar model with 3 equal units. 2 units are lost, 1 unit remains = 12 cards.
1 unit	=	12 cards
3 units (original)	=	12 x 3 = 36 cards