Use the following formula to calculate the slope os a line:
m = (y2 - y1)/(x2 - x1)
where (x1,y1) and (x2,y2) are any pair of points of the line.
Use, for instance:
(x1,y1) = (-7,7)
(x2,y2) = (4,-8)
and replace these values into the formula for m:
m = (-8 - 7)/(4 -(-7))
m = (-15)/(4 + 7)
m = -15/11
Hence, the slope of the given line is -15/11