Let's assume the average speed of a serve in men's tennis is around 135 mph, with a standard deviation of 6.5mph. Because these statistics are calculated over many years and many players, we will treat them as population parameters. We develop a new training method that will increase arm strength, the force of the tennis swing, and the speed of the serve, we hope. We recruit 9 professional tennis players to use our method. After 6 months, we test the speed of their serves and compute an average of 138 mph.

To test the hypothesis that our method makes a difference, calculate a 95% confidence interval. List the lower bound (rounded to 2 decimal places):