Specify critical region, calculate the test stat?

Ok the question says:

Specify the critical region and calculate the test statistic based on data in the table below:

Ho: =75

Ha: < 75

alpha = .05

n = 64

=72.7

= 8.6

Here is what I have come up with, I am really confused so I'm not even sure if it's relevant.

z' = - (all over) / square root n

z' = -2.1395

With z-test I found that p = .0162

I'm not sure where to go from here or if I'm even heading in the right direction. Please and thank you for the help (Happy)