Hypothesis testing? Please help with this question!

Can someone kindly walk me through how to solve this question? I'm very confused

Priorto a recent increase in advertising by one of its competitors, the dailysales of t-shirts at a particular clothing store have been observed to follow a nearNormal distribution with a mean of 500 t-shirts and a standard deviation of 50t-shirts. In the 35 days since the increase in advertising, the mean daily sales hasbeen 475 t-shirts.Carry out a test to see whether there hasbeen a decrease in the daily sales of t-t-shirts since the competitor’s increase in advertising, clearly stating your null and alternative hypotheses. Use a 5%significance level. Justify your choice of test and state your conclusionclearly.