Step-by-step explanation:
the slope of a line is the ratio y/x.
that means when going from one point to the other, how many units is y changing, when x changes a certain number of units.
so, going from one point to the other here, x changes by 30 units (from -12 to +18). and y changes 15 units (from -9 to 6).
therefore, the slope is
15/30 = 1/2
