Lau kasle kahile padheko thaha chhaina tara let me explain where it is wrong : when you take the roots { Sq. Rt( 4-(9/2))^2} , there are + and - values on both side . after that its not you who take either positive or negatinve on both side . You take the sign on one side and its mathematics that guides what sign you have to take on other side. Well any way this did remind me of my school days and do keep on throwing these memories of past.