 Let's start with delta xa. So Alice measures the time delta ta between these two events and in this time She's been traveling at V However the first event from Alice's point of view happens at her origin now The second event happens behind her so delta xa is x2 minus x1 Which will give us minus V delta ta now in Bob's frame both of these events happen at the same place the origin So delta xb equals zero finally how do delta ta and delta tb relate? So now these two events happen at the same place in Bob's reference frame So now we have delta ta equals gamma delta tb So now we can substitute these values in and solve for c and e but in this case We have to do a little bit more algebra. So as a hint First show that one on gamma minus gamma is one minus gamma squared over gamma And then one minus gamma squared is minus V squared on c squared gamma squared Why gamma? It's our Lorentz factor. These identities should make your calculations a little bit easier