To find the slope of a line you must do

Look at the image to find the rise (in pink) and run (in green) on the graph:
In this case the rise is 3 and the run is -4 (the reason the four is negative is because it "runs" to the left, which is the negative direction of the coordinate plane) so the slope of this line is:

Hope this helped!
~Just a girl in love with Shawn Mendes