Prove that y y, where y _ Aly with A being the matrix that transforms observations into deviations from sample means , is the RSS when y is...

Hello, I have a lot of difficulties with this exercise about linear regression model (with matrices).

Is there someone who can explain to me how can I solve it?

5. Prove that y y, where y _ Aly with A being the matrix that transformsobservations into deviations from sample means , is the RSS when y isIFETESSEd an In = 1. Show also that the estimated coefficient of theFEETESSION is* _