# Special relativity and Lorentz factor problem

## Homework Statement

There are 2 particles(1,2) separated ∆x=L moving with the same velocity u_x in frame of reference S , there's an other reference S' moving at v .
I have to calculate ∆x'. GAMMA(LORENTZ'S FACTOR)

## The Attempt at a Solution

I have done x1=0 when t=0
So for particle 1 x1=ut
Particle 2 x2=L+ut
So ∆x=L=gamma(∆x') , and it's not correct
The solution I have got from class is
X1=x0+ut1
X2=x0+L+ut2
So when I measure the distance between them in S' the solution is completely different, the thing I don't understand is , how t2=\t1 and not t2=t1.