Friction question!

1. The problem statement, all variables and given/known data

A child sits on a freshly oiled, straight stair rail that is frictionless. She has a mass of 25 kg and the rail makes a 40 degree angle with the ground. IF she slides 4.0 m before reaching the bottom, what is her speed there?

2. Relevant equations

f=ma

3. The attempt at a solution

I did .1(9.8)(sin30)
then i set it equal to .1a=.1(9.8)sin30
and got 4.9 m/s/s but the actual answer is 7.1 m/s/s?
where did i go wrong?
 
Firstly, if it is frictionless then it is not a friction problem lol. What other equation do you think you need to solve this?
 

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