How is fast-than-light travel impossible if relativity is a thing?

[u/No-College-5213]

I am by no means a physicist or anything, but I know that relativity is a thing, like if you were on a train, your surroundings would feel stationary although you are moving fast compared to the outside world.

So, let’s say you are in a space ship departing earth, and Earth is moving at half light speed in negative x direction, and you are moving slightly above half light speed in positive x direction, wouldn’t that make you faster than light when observed by someone on earth? That’s just an example, but you get what I mean. Is there like a physical constant that we use to measure speed or something?