It is very simple to calculate slope of a when you have two pair of points. You can use the following formula.

M=(y2-y1)/(x2-x1)

Here the two points are

(8,-1) and (4,3)

so

m =(3+1)/(4-8)

m=4/-4

m=-1 is the required slope of the line.

M=(y2-y1)/(x2-x1)

Here the two points are

(8,-1) and (4,3)

so

m =(3+1)/(4-8)

m=4/-4

m=-1 is the required slope of the line.