just to add some more to matthewwjohnson92 great reply above.
part A
year........value
1...........25000 - 1200(1)
2...........25000 - 1200(2)
3...........25000 - 1200(3)
4...........25000 - 1200(4)
x...........25000 - 1200(x)
<h2>y = 25000 - 1200x</h2>
part B
Check the picture below.
how much lost each year? from 0 to 3 it lost 9000, so 9000/3 = 3000 per each year.