Hi,
Here is the solution.
3 years = 36 months
Leased amount for 3 years = $3000 + 36*350
= $3000 + $12600
Leased amount for 3 years = $15600
Residual value = $12000
Assume you didn't buy the car, you took it for lease, so your expense = $15600.
Now Assume you bought the car for $21,000.
After using 3 years, you can sell it for $12000.
Your expense = $21,000 - $12,000 = $9000.
Conclusion:
It is less expensive to buy the car.
The cost for buying the car and selling it after 3 years = $9000.
Hope you will understand the steps. Thank you :)