Rise over run, -2/4 or -1/2 so the slope is -1/2 or -0.5. (View picture) And the slope is negative because you are going down wards \ and the "rise" is going down.
The answer would be -1/4 because the slope is the number before x but a slope has to be a fraction because the slope is also rise over run and if that does not make sense I can explain it differently.
