secondary 3 chevron_right E Maths chevron_right Singapore

I really do not understand this equation I understand for the second line we divide all by 2 but shouldn’t we use the cross multiplication method for the second line? Why is it seemed to be using one of the special products and for the fourth line I do not understand how there is a (x+4)^2 and shouldn’t all answers be something OR something why is there only one answer to this equation someone please help and explain super clearly for me because I really don’t get this equation thank you very much for help