 Let's start with yB. All the motion is along the x-axis, and so it shouldn't be too hard to convince yourself that yB is just yA. Alice and Bob agree on the y-coordinate. Okay, what about the x-coordinate? Well, this is at some time t, and all this time Alice has been travelling at a speed v. So overall, the distance Alice has travelled in this time is vt. And from this, you should be able to convince yourself that xB equals xA plus vt. These are what we call the Galilean transformations between two inertial reference frames.