So, I'm going to break it down to help you understand it a bit more.
If it starts at (0,-2) and crosses through (1,0) that means it moved to the right once and up twice. Which means, that the slope will be 2. If it were one it would be to the right 1 up one, if it were 4 it would be to the right 1 up 4, and finally if it were 1/2 it would be to the right 2 up 1.
