Use the following formula for the slope of a line:
m = (y2 - y1)/(x2 - x1)
where (x1,y1) and (x2,y2) are two points of the line.
You have:
(x1,y1) = (-1,4)
(x2,y2) = (1,-2)
replace the previous values into the formula for m:
m = (-2 - 4)/(1 - (-1))
m = (-6)/(1 + 1)
m = -6/2
m = -3
Hence, the slope of the line is m = -3