do I use t or z?

Question:
Calculate the following:xbar, where P(Mu > xbar) = 0.025, df = 20, Muxbar = 39.4, sigmaxbar = 28.7

My attempt:
I guess that I can plug muxbar, t =2.086 using my table into t = (xbar - mu)/(s/sqrt n) but I don't know about s.

muxbar is the mean of the samples so do I use the sigmaxbar since it's the SD of the mean of the samples? Is that the same as s? or can I not plug in SD??

thanks

z is for large sample
t is for small samples
The difference is usually taken to be around 30
When in doubnt, do it both ways and see if it makes any difference in your decision.