I need help

I need help
I have this homework that I cant solve it
Show that s^2=(1|n-2)∑(y-y^) is unbiased estimator for sigma^2 in linear regression model
can you help me??????
I am confused about the symbols
what do you mean by ybar
is ybar the mean value of y(y¯) or the estimated value of y?
and what do you mean by u_i is it the random errorƐ1) Y_i = Beta0 + Beta1X_i + u_i
and from where the relation (4) (4) e_i = (Y_i – Ybar) – b1(X_i – Xbar)
And how E[(b1 – Beta1)^2]= Var(b1)?
Thanks for help