Confidence Interval

In 22 patients with an unusual liver disease, the plasma alkaline phosphatase had a mean value of 39 IU/l (International Units per liter), standard deviation = 3.4 IU/l. Calculate the 95% confidence interval for the population’s true mean value of plasma alkaline phosphatase.

My equation

Step 1. Determine t*-value w/ t-table list.
T=22-1=21; t*=2.080
Step 2.Solve for x, s, and n
X=39IU/1, S=3.4 IU/1 n=22
Step 3. Margin of error
Confidence Interval: 39+- 1.51; Range for true population mean:37.49 to 40.51

I'm curious if this is the correct formula.