Car Deappreciation vs Kilometers done

Hi, i'm really disappointed with myself, i'm a 2nd year math student in university, and tutoring a 7th grader, and i couldnt figure this out.

I have a graph, with car's resale value on y-axis, and kilometers done on it on x-axis.

There's 2 linear relationships given, between x and y.

Then it asks about the number of kilometers on the car, when its resale value is $0.

The two lines form a continuous graph, but it's not perfectly a straight line, (The 2 have different equations). Then they stop at some point, and asks for x when y = 0.

Is it a curve fitting problem, and if so... is that taught in 7th grade, i have no clue. Because i have to use techniques that are covered in his school, and my damn problem is that his textbook is in french. (in canada)