If the cart is moving with a velocity of magnitude 29.11m/s at the top of the loop-de-loop, what must be the magnitude of the normal force from track on the cart at the top?

The Normal Force is zero.

Explanation:

The reason for the normal for to be zero is because no matter if the is a form of contact at the top, the is no force applied by the cart to loop-de-loop, and so vice versa.

If the cart was on the bottom, then the normal force would be equal to the weight of the cart, which would be te force the cart would be applying to the loo-de-loop, and therefore by newton's third law, the loop-de-loop would apply an equal force to the cart, which we would say is the normal force.