5.46. a)
Let's sum the forces in the x direction (along the track) as if the car was just about to slip down the plane.
Summing the forces in the y direction yields
Now, since 
, we find that
Plugging this in we find
Solving the above for the velocity yields
If the car slips up the plane then the frictional force is in the opposite direction, so the sum of the forces in the x direction yields
meanwhile, the sum of the forces in the y direction remains
unchanged, so we find that when we plug in for the frictional force
and solve for the velocity, this time 
, we get
b)
If the minimum speed is zero, then
which is only zero when
c)
