What is the Formula for Calculating Force of Friction on an Inclined Surface?

[Jus' me]

For the question: A 435 n box is sliding down a 40.00 inclined. If the acceleration of the box is 0.250m/s2. Which formula would I use to find just the force of friction? and would I change the Newtons to kilograms?

 

[Fun Value]

The mass of the box is m = Fw/g = 435n / 9.81 m/s2 = 44.3 kg.
Is it 40 degrees?
The parallel force Fp = Fw(sin40) = 280n
The net unbalanced force responsible for acceleration is F = m a = 44.3kg (0.25m/s/s) = 11.1n.
If Ff = |Fp| then the box would not accelerate.
The Force of friction, Ff = 280n - 11.1n = 268.9n I think, but correct me if I am wrong.
Thanks.

 

[Jus' me]

Well, the answer is supposed to be 10 but I think there was probably a typo. :)
thanks so much for your help!
and yes, it was supposed to be 40.0 degrees. :)