Probabilities- Statisctics

[Guest]

Hello guys, I recently started my course on statistics and probabilities but I am helpless with the problem below so any help or small tip is highly appreciated!

In an experiment with a new tranquilizer, the pulse rates (per
minute) of 12 patients were determined before they were given
the tranquilizer and again 5 minutes later, and their pulse rates
were found to be reduced on the average by 7.2 beats with a
standard deviation of 1.8. At the level of significance 0.05,
do we have significant evidence that the mean pulse reduction
with this tranquilizer is less than 9.0 beats?

Thanks in advance!