posted by djspidal .
A roller-coster car starts from rest at the top of a 16 meter hight track, and slides without friction through a 6 meter tall loop. Find the speed of the car at the bottom of the loop. find the speed of the car when it's upside down at the top of the loop
The answer depends upon the elevation of the 6 meter loop, relative to the starting elevation. That is not clear from your decription.
In any case, use conservation of energy.
By the way, not all cars in a roller coaster "train" are at the same altitude most of the time. It is the CM elevation of the entire train that matters