Answer:
8
Step-by-step explanation:
goes plus 1 then plus 2 then plus 3 so then it would be plus 4
When x=-1:

Ok that gives us a little more information.
If we implicitly differentiate with respect to t, from the very start, then we can apply our product rule, ya?

The right side is zero, derivative of a constant is zero.
Where x' is dx/dt and y' is dy/dt.
From here, plug in all the stuff you know:
y' = -3
x = -1
y = 4
and solve for x'.
Hope that helps!
Answer:
y=-2 is the correct answer...
According to my calculations, after 200 miles, the prices will be equal.