Answer:
a) In order to check if an estimator is unbiased we need to check this condition:

And we can find the expected value of each estimator like this:
![E(\theta_1 ) = \frac{1}{7} E(X_1 +X_2 +... +X_7) = \frac{1}{7} [E(X_1) +E(X_2) +....+E(X_7)]= \frac{1}{7} 7\mu= \mu](https://tex.z-dn.net/?f=%20E%28%5Ctheta_1%20%29%20%3D%20%5Cfrac%7B1%7D%7B7%7D%20E%28X_1%20%2BX_2%20%2B...%20%2BX_7%29%20%3D%20%5Cfrac%7B1%7D%7B7%7D%20%5BE%28X_1%29%20%2BE%28X_2%29%20%2B....%2BE%28X_7%29%5D%3D%20%5Cfrac%7B1%7D%7B7%7D%207%5Cmu%3D%20%5Cmu)
So then we conclude that
is unbiased.
For the second estimator we have this:
![E(\theta_2) = \frac{1}{2} [2E(X_1) -E(X_3) +E(X_5)]=\frac{1}{2} [2\mu -\mu +\mu] = \frac{1}{2} [2\mu]= \mu](https://tex.z-dn.net/?f=%20E%28%5Ctheta_2%29%20%3D%20%5Cfrac%7B1%7D%7B2%7D%20%5B2E%28X_1%29%20-E%28X_3%29%20%2BE%28X_5%29%5D%3D%5Cfrac%7B1%7D%7B2%7D%20%5B2%5Cmu%20-%5Cmu%20%2B%5Cmu%5D%20%3D%20%5Cfrac%7B1%7D%7B2%7D%20%5B2%5Cmu%5D%3D%20%5Cmu)
And then we conclude that
is unbiaed too.
b) For this case first we need to find the variance of each estimator:

And for the second estimator we have this:

And the relative efficiency is given by:

Step-by-step explanation:
For this case we assume that we have a random sample given by:
and each 
Part a
In order to check if an estimator is unbiased we need to check this condition:

And we can find the expected value of each estimator like this:
![E(\theta_1 ) = \frac{1}{7} E(X_1 +X_2 +... +X_7) = \frac{1}{7} [E(X_1) +E(X_2) +....+E(X_7)]= \frac{1}{7} 7\mu= \mu](https://tex.z-dn.net/?f=%20E%28%5Ctheta_1%20%29%20%3D%20%5Cfrac%7B1%7D%7B7%7D%20E%28X_1%20%2BX_2%20%2B...%20%2BX_7%29%20%3D%20%5Cfrac%7B1%7D%7B7%7D%20%5BE%28X_1%29%20%2BE%28X_2%29%20%2B....%2BE%28X_7%29%5D%3D%20%5Cfrac%7B1%7D%7B7%7D%207%5Cmu%3D%20%5Cmu)
So then we conclude that
is unbiased.
For the second estimator we have this:
![E(\theta_2) = \frac{1}{2} [2E(X_1) -E(X_3) +E(X_5)]=\frac{1}{2} [2\mu -\mu +\mu] = \frac{1}{2} [2\mu]= \mu](https://tex.z-dn.net/?f=%20E%28%5Ctheta_2%29%20%3D%20%5Cfrac%7B1%7D%7B2%7D%20%5B2E%28X_1%29%20-E%28X_3%29%20%2BE%28X_5%29%5D%3D%5Cfrac%7B1%7D%7B2%7D%20%5B2%5Cmu%20-%5Cmu%20%2B%5Cmu%5D%20%3D%20%5Cfrac%7B1%7D%7B2%7D%20%5B2%5Cmu%5D%3D%20%5Cmu)
And then we conclude that
is unbiaed too.
Part b
For this case first we need to find the variance of each estimator:

And for the second estimator we have this:

And the relative efficiency is given by:
