Find the interval [ z / n + z / n] within

Find the interval [μ– z σ / √ n μ + z σ / √ n] within which 90 percent of the sample means would be expected to fall, assuming that each sample is from a normal population.
a. μ = 100, σ = 12, n = 36
b. μ = 2,000 σ = 150, n = 9
c. μ = 500, σ = 10, n = 25