When you are given two points, you can find the slope by finding change in y's and dividing it by change in x's.
In terms of formulas:
Slope is
[tex]m=\frac{y_2-y_1}{x_2-x_1}[/tex]Where x_1, x_2, y_1, and y_2 are the respective coordinates.
The coordinates are:
(3,4) and (1,8)
Putting into formula:
[tex]Slope(m)=\frac{y_2-y_1}{x_2-x_1}=\frac{8-4}{1-3}=\frac{4}{-2}=-2[/tex]