The carriage has potential energy since the carriage has the potential to fall and convert that potential energy into kinetic energy.
The potential energy in this case is 15*36 = 540 joules.
The potential energy formula is
PE = m*g*h
where mg is the weight of the object, so you can express it like this
PE = w*h
So that's why we multiply the weight with the height to get the potential energy.
