The rule of the slope of the line is
(x1, y1) and (x2, y2) are two points on the line
Since the line passes through points (-1, 8) and (12, -5), then
x1 = -1 and y1 = 8
x2 = 12 and y2 = -5
Substitute theses values in the rule above to find m
The slope of the line is -1