You can do this two different ways. Both include picking two points where the line crosses a corner. I'm using (0,-1) and (4,2). Now here you can either use
Slope=
or
If you use the first one start at point (0,-1) and go to point (4,2) and count how many it goes up (rise) and then put that over how many it goes to the right (run).
If you use the second one then plug in the numbers.