albuino:
V = 24 VDC, R = 12 Ohm.
So,
I = 2 A, P = 48 W.
An almost 50 W resistor... ¿¿¿??? It's too much!!!
No, it's correct. You need one of these: http://www.ebay.com/itm/250911320379
(Although that's going to get very hot at full power so you should really get 100W. Maybe 150W to be safe...)