A 71.8-kg skier coasts up a snow-covered hill that makes an angle of 25.1 ° with the horizontal. The initial speed of the skier is 8.84 m/s. After coasting a distance of 2.10 m up the slope, the speed of the skier is 3.41 m/s. (a) Find the work done by the kinetic frictional force that acts on the skis. (b) What is the magnitude of the kinetic frictional force?

Part a= -1761 J

I Just cant figure out how to find part b

$$W=r \cdot F$$