@Jeronimus Thank you Jeronimus , I figured my mistake . I was presuming t' = t , which was wrong . And you example really helped me .
@Ibix Will try to be more clear next time . Thanks .
I recently saw the derivation of length contraction in Special Relativity . At the end , it said
x' = (x - vt) γ(gamma)
x = (x' + vt') γ(gamma)
Where γ(gamma) is Lorentz transformation . It is = 1/√(1- v²/c²)
Then derivation continued , with expansion of x' = (x + vt)γ
As t = 0 in this case...