X1 and S2/1 are the sample mean and sample variance from a population with mean and variance

X1 and S2/1 are the sample mean and sample variance from a population with mean μ and variance similarly, and are the sample mean and sample variance from a second independent population with mean μ1 and variance σ2/2. The sample sizes are n1 and n2, respectively.
(a) Show that X1 – X2 is an unbiased estimator of μ.
(b) Find the standard error of X1 – X2. How could you estimate the standard error?