Final velocity problem involving a roller coaster changing height

[AHUGEMUSHROOM]

A roller coaster (475kg) passes point C with a velocity of 10.0 ms-1, calculate the velocity of the roller coaster at point A.

mass = 475 kg
velocity at C = 10 ms-1
height at A = 30m
height at B = 0m
height at C = 25m
height at D = 12m

Ep = mgh
Ek = 1/2mv^2

The trolley is moving from Point C towards point A.

*My method of finding out the answer was incorrect so you might want to ignore the following*

I substituted the data given into Ep = mph (h at point A) and got Ep = 139,650J

Then said Ep = Ek and subbed the data into Ek and got final velocity = 24.5 ms-1


*THE CORRECT ANSWER IS THE SQUARE ROOT OF TWO*

 

[ehild]

Apply the conservation of mechanical energy. What does it say?

ehild

 

[AHUGEMUSHROOM]

I'm sorry but we have not come across mechanical energy yet and I am sure it was not used in working out the answer.

 

[ehild]

So what are Ep and Ek?

ehild