Statistics, estimator of consistency

Suppose that X sub 1, X sub 2,... X sub n and Y sub 1, Y sub 2,... Y sub n are independent random samples from populations with means Mu sub 1 and Mu sub 2 and Variances Sigma sub 1 squared and Sigma sub 2 squared, respectively. Show that X bar - Y bar is a consistent estimator of Mu sub 1 - Mu sub 2.