[Econometrics] OLS when intercept is 0

[Error404pnf]

Hi,

I'm studying with the book "Introductory Econometrics, A Modern Approach" (Wooldridge, 5th edition) and I am stuck at the problem 8 from chapter 2. The question is:

Consider the standard simple regression model y = b0 + b1 x + u under the Gauss-Markov Assumptions SLR.1 through SLR.5. The usual OLS estimators bˆ0 and bˆ1 are unbiased for their respective population parameters. Let b˜1 be the estimator of b1 obtained by assuming the intercept is zero.
(i) Find E(b˜1) in terms of the x i , b0 , and b1 . Verify that b˜1 is unbiased for b1 when the population intercept (b0 ) is zero. Are there other cases where b˜1 is unbiased?
(ii) Find the variance of b˜1 . (Hint: The variance does not depend on b0 .)

I found the answer for question (i) but not for the question (ii). The final solution to the question (i) and the first step for question (ii) are in the files attached to this post.

Could anyone explain to me the first step of the question (ii) ? I don't get how I can find the variance from the answer in part (i).

Best regards !