Finding normal force acting on an object (Fn)?

[shamma]

1. A couch with a mass of 100 kg is placed on an adjustable ramp connected to a truck.If the couch slides down the ramp with an acceleration of 0.8 m/s^2:

2. find the normal force on the couch

3. my way:
Fg=m*9.81=981
Fnety=0N
981 sin 270+f sin 180+Fn sin 90=0N
-981Fn=0
Fn=981
my ans is wrong

hint:the ans shold be (Fn=959.6N)

 

[rude man]

Hmm - I get Fn = ma*cot[arc sin(a/g) = 977.73 N

Assumes frictionless plane