"A hockey puck is hit on a frozen lake and starts moving with a speed of 12 m/s. Exactly 5.0 s later, its speed is 6.0 m/s. What's the puck's average acceleration? What is the coefficient of kinetic friction between the puck and the ice?" I've calculated the acceleration, but I'm stuck when it comes to finding the coefficient of kinetic friction since I don't know the mass of the object.