If you add a third point, c = (2, -1), and use it to form a triangle with the other two ones, you’ll notice that the slope of ab equals tg (180º - â).

Maybe some people won’t see the character after 180º right, it’s an ‘a’ with a ^ on top of it. Also, I put the 180º there because the line is decreasing. If it weren’t, it’d be tg â.

From what I recall, this solution is the very definition of ‘slope’. The only reason I’m not 100% sure to say it is due to me not knowing math terminology in English so well, so please correct me if I made any mistakes.

Difficulty is in the eyes of the beholder. There are people (me included) who find it a lot easier to do it the way I posted, because once you get the hang of it you can solve that problem in no time. See, when I looked at it, I pictured the triangle in my mind (I’m so used to it, it comes as an instinct already) and just thought “it’s 2 / [20^(1/2)]”. Took me a few seconds. After taking more time to elaborate we get {1 / [5^(1/2)]}.

There are many people who find the whe other way easier. If you remember the formula provided by Nightblade, you can solve most problems even faster. But I have a hard time memorizing formulas. I find it easier memorizing the definitions and making my way through them. In this case, the definition for slope of a line (if slope is what I’m thinking it is) is the tangent of the angle between the X axis and the right side of the line in question. And the triangle I cite has its hipotenusa over that line.