okay so this is event, the event that i'll call event number one and this here is event number two, event number two is the instant when the right end of the ladder is about emerge from the garage and that occurs of course at the position x^2 is equal to g as seen by the, by the garage frame and of course at that same, at that same instant, at that event, is the event where the world line of x prime equals l which is this blue line, the motion of the right side of the garage intersect x2 = g.