Explanation:
Plug x = 0 into the first equation to find that C = -3. We use x = 0 since 0 years have passed by (the starting point is 2005 for this equation).
Now plug x = 0 into the second equation. The starting point is now 2010 which explains why we use the same x value, just for a different equation. You should get C = -2 here.
The difference from C = -3 to C = -2 is 1, as this is the distance between the two values.
Chances are C is measured in thousands of dollars, so C = 1 represents an average cost of 1000 dollars. Though your teacher never mentions "in thousands of dollars", so it's probably best to stick to 1 instead of 1000. I would ask your teacher to clarify.
