Question

A 75 kg water skier is pulled by a horizontal tow rope. She goes from rest to 6.0 m/s in 4.5 s when the rope's tension is 140 N. What is the magnitude of the friction force between her skis and the water?

I really dont know where to start, can someone please help :)
oobleck
in the absence of friction,
F = ma, so she would only need 75*(6/4.5) = 100N
so the rest (40N) is friction