Hey homalina.

Hint: what is the distribution of (X_bar - Y_bar) - (mu1 - mu2) going to be? You already know the variances and that they come from a normal distribution, and any linear combination of normal distributions gives back a normal.

With this information, what is the distribution of (X_bar - Y_Bar) and how do you standardize it? (Another hint: you assume mu1 - mu2 = 0).