Hamendra,
The problem is towards the end when you take the square root and equate.
square(A) = sqaure(B) only means that |A| = |B| (i.e. either A=B or A=-B)
and not A = B ( for example square of 2 and -2 is same).
In the case you have taken, A = -B is correct (A=3-5/2 and...