THe slope of a line is defined as the rise / run - for every step you take along the horizontal line, how many steps do you move up the vertical line.
In the case of our graph, let us see by how much we have to move horizontally and vertically to get to another point on the line.
The graph shows that for every 4 steps we move vertically down, we have to move 2 steps to the right in order to get to another point on the line; therefore, the slope of the line is
which is our answer!