Computing the test statistic: Not sure which Formula I need

#1
Not sure which formula to use...

A Health and Fitness company would like to determine if a new product improves cardiovascular fitness. A sample of 20 new customers agreed to take a treadmill test before and after using the product. The test measured how long they could run on the treadmill until they became exhausted or short of breath. The mean difference (after - before) for the customers was computed to be 7.3 minutes with a standard deviation of 6.7 minutes. Using a two-tailed test compute the test statistic and make the decision.

I tried using Paired data Formula, that didn't seem to work. Thanks for help :)