A random sampling of a company's monthly operating expenses for n = 36 months produced a sample mean of $5938 and a standard deviation of $766. Find a 90% upper confidence bound for the company's mean

A random sampling of a company's monthly operating expenses for n = 36 months produced a sample mean of $5938 and a standard deviation of $766. Find a 90% upper confidence bound for the company's mean monthly expenses. (Round your answer to two decimal places.)

I have tried doing: 5938 + 1.645 * (766 / sqrt(36)) = 6148.01 and 5938 - 1.645 * (766 / sqrt(36)) = 5727.99 but these are incorrect. I don't know what else to try.

The professor never taught us how to do upper confidence bounds so I have no idea how else I could do this question.