I'm reading about special relativity and looking at the Lorentz transformations.

I'm reading that:

If $v > c$, we can find a frame in which $t_2' < t_1'$, i.e. a signal arrived before being sent.

I am trying to satisfy myself that that is true. I am beginning with the invariant spacetime interval:

$c\Delta t^{2} - \Delta x^{2} = c\Delta t'^{2} - \Delta x'^{2}$

I assume that I'm trying to show that $\Delta t' < 0$, but I'm not seeing how to do that.