Standard Deviation for the Difference of Means

A random sample of size n₁ = 25 is selected from a normal population with a mean μ₁ = 75 and standard deviation σ₁ = 12. An independent random sample of size n₂ = 16 is selected from a second normal population with a mean μ₂ = 70 and standard deviation σ₂ = 8. What is the standard deviation of the sampling distribution of the difference between the sample means, x̄₁ − x̄₂?