Essentially, you can always tell a thing's position by 3 coordinates. We also say that our world is three-dimentional: it has three dimensions -that's why three coordinates are needed. If something is on a flat surface, you know its one coordinate (all points on the same surface have the same coordinate) so you only need two more (you can call them vertical and horizontal)
The slope between (5,1) and (2,-7) is 8 over 3
There are 27 cats if there are 45 dogs
