Unbiased estimators?? Can't do it.

Given X1,...,X7, a random sample from a population of mean mu and variance sigma^2. Prove that the following estimators are unbiased:

estimator 1 = (X1+...X7)/7 = mean = unbiased by definition,

my problem is with the second one:

estimator 2 = (2X1 - X6 + X4) / 2

Then they ask which estimator is the best, so that I kinda know, it will be the estimator with the lower variance but I can't solve for the variance because I have no numbers.

And then they ask for efficiency...needless to tell that I don't know that 1 either.

Much appreciate it guys!!