Can anyone elaborate more about this:

=

[Math]\sum_{i=1}^nVar[(Y_i-\overline{Y}-b_1(X_i-\overline{X}))^2][/tex]

How is the 2nd term combine into the variance?

To be more specified: This is the intermediate step in proving [LaTeX ERROR: Convert failed] in regression study. furthermore, [LaTeX ERROR: Convert failed]