 If we take this suggestive form and look at the term on the right and divide the top and the bottom by c squared, we get this. Pause the video now and give it a crack. If you're still having a bit of trouble, come back and watch for another hint. So the second term there we can write as gamma squared. So we have an expression for ta in red, an expression for tb in blue, and we have a relation between ta and tb in black. See if you can combine these three.