Least Square Estimators - Variance of Estimators, b0 and b1, Proof

** NOTE: At minute 11:48, I forgot to write the "squared" above X-bar. Later, at minute 28:26, when I summarize everything I found and solve for Var(b0), I do have X-bar squared. This was a mistake at minute 11:48, though it was not a mistake that I carried to my solution (the solution at the end of the video is correct).
The centered model is referenced in this video; here is a link with more discussion of the centered model: • Simple Linear Regressi...