How do you calculate MOE given Mean/Standard Deviation/Confidence Interval?

Question

An example problem:

You measure 40 turtles' weights, and find they have a mean weight of 51 ounces. Assume the population standard deviation is 13.4 ounces. Based on this, what is the maximal margin of error associated with a 90% confidence interval for the true population mean turtle weight.

I've tried Khan Academy, Lumen Learning, Youtube, and coworkers, none of which seemed to help much; I think I have no problem with the calculations until I get an incorrect answer, and then I'm unable to tell what I'm doing wrong where. I've tried using both Z and T charts, as well as all variety of relevant formulas, but I must be missing something.

My professor has yet to respond to emails, and my assignments are due in 5 hours. If anyone could do a semi-detailed walkthrough of even just this one problem, it would go a long way. Excel or by hand doesn't matter, I just don't own a TI-83/84/a science calculator (aside from iphone).

Answer 1

margin of error = z-critical * standard deviation/square root(sample size)

z-critical for 90% CI is 1.645

you are given standard deviation is 13.4 and sample size is 40.