determine the critical region, given critical value, etc.

I have a critical value of -1.645 and need to determine if I am able to rejet the null hypothesis. I am not sure how to do this. My null hypothesis is 12.0 and the mean is 11.9 there is a standard deviation of .4 and a type 1 error of .05 how do I determine this or a formula or something.
Thanks a bunch.

You didn't mention a sample size, among other things. If you are testing a hypothesis you should know how to compute a z-score for this problem.

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I know what my z score is its -.25 my sample size is 30 I believe that I should not reject the null hypothesis but Im not sure how to refer that to the critical value.