A schoolteacher is worried that the concentration of dangerous, cancer-causing radon gas in her classroom is greater than the safe level of 7 pCi/L. The school samples the air for 16 days and finds an average concentration of 7.4 pCi/L with a standard deviation of 1 pCi/L. Calculate the value of the test statistic.

So I believe from what I've learned so far that I have the correct formula, but somehow I'm plugging the numbers in wrong.