The requested equation is
C(m) = 7.50 + 0.25*m,
where C(m) is the total cost (in dollars) for "m" miles.
Make the plot on your own.
The answer to the question is 2
The tangent of the angle is
... tan(α) = ay/ax = -8.6/6.1
Then the angle (measured CCW from +x) is
... α = arctan(-86/61) ≈ 305°