 So the photon is moving diagonally at speed c. So where does the horizontal motion come from? Well, this is just the light clock being carried along by the train, which is moving at speed v. So the horizontal component of its motion is just v. And then just from the Pythagoras theorem, the vertical motion is the square root of c squared minus v squared. And the situation is similar for when the photon is coming back downwards. Okay? So then the total distance the photon travels in the up and down direction is 2L. And its velocity in the vertical direction is root c squared minus v squared. So now we have an expression for tb. Can you write this in terms of ta?