Standard Equation of a line is:

Where m is the slope and b is the y-intercept.

The first thing to do is find the slope. Recall the slope formula:

In this case we know that when the x variable has changed 1 unit, y has decreased by 1,200. So, the slope is:

We also need the y intercept. This is at x=0. We know that the car is originally worth 26,000 dollars so our y intercept is:

Plug it in to our standard equation to get:

The car will be worth $500 when so just set the equation equal to 500:

And solve for x.