Explanation:
the slope of the line is expressed as the ratio
y coordinate change / x coordinate change
when going from one point to the other.
the trick in such a situation is to find good points on the chart that are crossed by the line. and by "good" I mean points with round integer coordinates.
I see 2 such points :
(-2, 3) and (0, -3)
x changes by +2 (from -2 to 0)
y changes by -6 (from 3 to -3)
the slope is -6/+2 = -6/2 = -3