ABC purchases a copier for 30,000. ABC estimates useful life and salvage value are for 10 years and 6,000. ABC uses straight line depreciation method for accouting. After 4 years, the machine is sold for 17,000. What gain or loss should be record and how?

Compare the net book value with sold price at the time of the sale.

Dr. cash, accumulated depreciation
Cr. Asset
Dr loss or Cr gain

Machine cost: 30,000
Minus Salvage: 6000
Balance: 24,000

Straight line method of depreciation means that every year the amount of the depreciation is the same, so:
24,000/ 10 years = $2,400 depreciation expense for one year
$2,400 * 4 years = $9,600 total accumulated depreciation for 4 years.

The value of the machine after 4 years is: 24,000 - 9,600 = $14,400.

Sold for $17,000 so you have a gain of $2,600 on the sale of the machine.

$17,000 - $14,400 = $2,600.

Hope this helps

To get net book value should use cost minus accumulated depreciation, which is $20,400, loss of $3,400.