Okay so i'm having a little trouble explaining the answer to a phys lab question concerning rolling friction:

Assuming that f = µN , show that the coefficient of (rolling) friction for the car moving down the inclined plane with a constant speed is given by
µ = tanθ – (m₁ / m₂ cosθ).
(use symbols not numbers.)

I have no idea what m1 and m2 are

force down plane = m g sin T
normal force = m g cos T
friction force up plane = mu m g cos T

net force down plane = net force up plane
m g sin T = mu m g cos T
tan T = mu

Now if there is another mass and force in there, incorporate it