Answer
Each tick mark represents $0.5$ units.
Work Step by Step
Slope is the rise over run (or rise divided by the run).
Having a slope of $-\dfrac{2}{3}$ means that the rise is $-2$ and the run is $3$.
The two points have unknown y-coordinates, but their x-coordinates are known:
$(-2, y_1)$ and $(1, y_2)$
The run between the two points is $=1-(-2)=1+2=3$ (as expected)
The rise is represented by $y_2-y_1$.
Note that there are four tick marks from $y_1$ to $y_2$.
Since the four tick marks represent two units, then each tick mark must represent $0.5$ units.
