Business & farm
You may be able to find more information in the Car and Truck Expenses Worksheet which should report the depreciation component of the standard mileage rate as described by @HopeS above.
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I assume that line 21 is the cost less the depreciation to date equals basis. Presume there were no expenses of sale.
Line 22 is depreciation allowed or allowable which is recaptured. See Instructions for Form 4797 page 8 and 9 which says:
Line 22
Complete the following steps to figure the amount to enter on line 22.
Step 1
Add amounts such as the following.
• Deductions allowed or allowable for depreciation (including any special depreciation allowance...
