The slope would equal -1.
You would want to start with the first x coordinate (or lowest) so you would start with (1,2) and see what it takes to go to the next coordinate. You would then go to the right once and go down once from (1,2) to (2,1). Because you go to the right once and go DOWN once you could imagine that it is a negative slope because you’re going down. An easy way to do is run a straight line on a graph from (1,2) and (2,1) to easily view this.