Acceleration is defined as the rate of change of velocity. Force is the product of mass and acceleration.
The golf ball has a mass of 45 g. It is initially at rest and starts to move at a velocity of 25.9 m/s when it is struck with the golf club. When the golf club strikes the ball they are in contact with each other for 1.6 s.
The acceleration of the ball is given as: (25.9 - 0)/1.6 = 16.1875 m/s^2.
The average force applied by the club that results in the acceleration equal to mass*acceleration or (45/1000)*16.1875 = 0.7284 N.
The average force applied by the club while it strikes the ball is equal to 0.7284N.