the slope of the line is the gradient, which you can find through rise over run
m (gradient) = (y1 - y2) / (x1 - x2)
where (x1, y1) is the coordinate of the first point, and (x2, x2) is the coordinate of the second point
in your question:
x1 = -4
x2 = 19
y1 = -13
y2 = 11
m = (-13 -11) / (-4 -19) = -24 / -23 = 24/23 or 1.04 (2d.p.)
hope that helps :)