Hey kengil64.

One thing is that you can't cancel Sigma (x_i*y_i) / Sigma(x_i^2) the way you did it: you have to leave it like that. Cancelling terms like you do with normal fractions is not the same when you have summations.

You can only cancel things that are common terms outside any summation or multiplication and not inside (and if it's inside you have to show it's independent of the terms in the summation or multiplication and can be brought out).