Answer:
1. $3,700
2. $8,000 and $6,400
3. $5,120
Step-by-step explanation:
The computation of the depreciation expense for the years are shown below: 
1) Straight-line method: 
= (Original cost - residual value) ÷ (useful life) 
= ($40,000 - $3,000) ÷ (10 years) 
= ($37,000) ÷ (10 years) 
= $3,700
In this method, the depreciation is same for all the remaining useful life 
So for year 2019 and 2020 the same depreciation expense i.e $3,700 is charged separately for each year
(2) Double-declining balance method: 
First we have to find the depreciation rate which is given below: 
= One ÷ useful life 
= 1 ÷ 10 
= 10%
Now the rate is double So, 20% 
In year 2019, the original cost is 40,000, so the depreciation is $8,000 after applying the 20% depreciation rate 
And, in year 2020, the depreciation is 
= ($40,000 - $8,000) × 20%
= $6,400
3) For 2021, it would be 
= ($40,000 - $8,000 - $6,400) × 20%
= $5,120
Basically we applied the above formulas