 Okay. So now, if we want to represent the general coordinates of some event, we represent it like this. So these are what we call space-time coordinates. The first coordinate is the x, the second is the y, the third is the time. And lastly, the subscript a means this is in Alice's reference frame. So this would mean one meter along Alice's x-axis, three meters up Alice's y-axis, and half a second after the time Alice calls time zero. So here we have Bob. And Bob chucks a ball forward three meters per second relative to him. And just as he does that, he starts timing on a stopwatch. So that stopwatch measures his time axis. Now the ball is heading towards a wall, 13 and a half meters ahead of Bob. So in Bob's frame, what are the space-time coordinates of the event? A of the ball leaving his hand, B of where the ball is after two seconds, and C when the ball hits the wall. Assume that there's no gravity. The ball just travels straight ahead without falling. Now we also have Alice standing 20 meters in front of Bob. So now she's not moving relative to Bob. Alice starts her own stopwatch two seconds after Bob's. Again, what a A, B, and C, but this time in Alice's frame.