Differences of Random Variables

Hi guys, I need help with this question.

Suppose X has a normal distribution with mean u1 and known standard deviation 7.
Suppose Y has a normal distribution with mean u2 and known standard deviation 9.
Suppose we have a random sample of size 6 from the X distribution. The sample mean xbar is 24.
Suppose we have a random sample of size 8 from the Y distribution. The sample mean ybar is 27.

What is an appropriate value of zstar for a 95% confidence interval for u1 – u2?

Since the confidence interval has to be 95% is subtracted .95 from 1 to get .05 which is alpha. I then divided by 2. The z-score I got was 1.959964.

1-.95 = 0.05
0.5/2 = 0.025
z-score of 0.025 = 1.959964

Create a 95% confidence interval for u1 - u2.

24-27-1.959964*squareroot(49/6+81/8) and 24-27+1.959964*squareroot(49/6+81/8)

I did this and got (-11.382523 and 5.382523)

Calculate the variance of Xbar - Ybar

Now this third question is why I'm here. I have absolutely no clue how to solve this. Can someone offer me some help? Does anyone know the formula I can use to solve this and walk me through it?