Fastest snow sled for snow hill?

A girl is sliding down a snow covered 30 degree hill on a sled. Their combined weight is 30 N.?

• If their acceleration is 4.5 m/s^2 down the slope, what is the coefficient of kinetic friction between the sled and the snow covered hill? I know what the answer is, this is a multiple choice problem on a past test. I need to know step by step how to get the answer. The answer is 0.047 I can't seem to get it I get something else. THANKS! :)

• Answer:

  Use F = m*a where F is the combination of the component of weight down the ramp and friction acting up the ramp Now friction = µ*N where N = m*g*cos(θ) So we have m*g*sin(θ) - µ*m*g*cos(θ) = m*a so solving for µ we get µ = (g*sin(θ) - a)/(g*cos(θ)) = (9.80*sin(30) - 4.5)/(9.8*cos(30)) = 0.047