A hockey puck is hit on a frozen lake and starts moving with a speed of 12.0 m/s (more info inside)?

A hockey puck is hit on a frozen lake and starts moving with a speed of 12.0 m/s. Exactly 5.0 s later, its speed is 6.0 m/s. What is the puck's average acceleration? What is the coefficient of kinetic friction between the puck and the ice?