Ok, this should be an easy one but it's driving me nuts. When we take the Lorentz transformations and apply them to x2-c2t2 we get the exact same expression in another frame. I can do this math easily by letting c=1 and have seen others do it by letting c=1 but I have never seen anyone actually do it with the c's in there. It doesn't sound like it should be that hard but I just can't get it to work. Can anyone go through it?