You were fine up to
But then with the next step you have , which is incorrect. If you multiply it out you get:
In addition the step after that you go from to . This too is incorrect - if you were trying to divide both numerator and denominator by 'a' you would get .
Also, although you didn't ask - you have an error in the first one: check the sign of the term in the second line.
Ebaines - I actually meant for all of them to be checked so thank you for doing so. I think i have fixed the problem with number 4. Can you check again? I did it two different ways. I think they are both correct.