What am I missing?

Piper's Pizza sold baking equipment for $25,000. The equipment was originally purchased for $72,000, and depreciation through the date of sale totaled $51,000. What was the gain or loss on the sale of the equipment? Answer is not complete. Sale amount $ 25,000 Less: Cost of the baking equipment Book value 21,000 4,000 Gain on sale of the equipment $