Normal Force at a certain point in a loop

[mickjagger]

Homework Statement

A 5.4g mass is released from rest at C which has a height of 1.2m above the base of the loop-the-loop and a radius of .36m
Finde the normal force pressing on the track at A, where A is at the same level as the center of the loop. Answer in units of N

So the picture is of a ball (at point C) at the top of a 1.2m slope that goes down into the beginning of the loop the loop where point A is at 0^o from the center of the radius and point B is at the top of the loop.

Homework Equations

1/2mvf²+PE_i=1/2mv_f²+PE_f
Sum of all of the forces is = to ma_c or mass times the centripetal acceleration.

The Attempt at a Solution

I need to make and equation that works for this problem but I don't know where to start.

 

[mickjagger]

Top of loop elevation = 2 * 0.36 = 0.72 m
Top of track= 1.2 m

Figure change in energy from top of track to top of loop

PE = KE
MGH = 0.5MV^2
5.4 * 9.8 * 1.2 = 0.5*5.4 V^2

2.7 V^2 = 63.504
V^2 = 23.52
V = 4.85 m/s

Fn = Force of Gravity - Centripetal Force
Fn = MG - M* (V^2/r)
Fn = 5.4 * 9.,8 - 5.4 *(4.85^2 / 0.36)
Fn = 5.4 * 9.8 - 352.8 = 52.92 - 352.8 =299.88 N That is the force of the track down against the ball

It seems as if this isn't the answer my prof is looking for. can anyone tell me where I went wrong. I have an hour left to get this problem so I'm frantic right now.