If each year the computer loses $200 of it's value, in 3 years it'll have lost 3 times that amount i.e. $600.
So, after 3 years the computer will have $600 less the value than when Tim bought it:
After 3 years, the value of the computer will be $600
The loss of the computer's value can be model as:
Where 1200 is the initial value, 200 is how much it's devalued per year. 'x' represents the years since Tim bought the computer and 'y' represents it's value after 'x' years.