To determine the slope of a line you need to know at least two points of the line.
The formula to determine the slope is:

Where
(x₁,y₁) are the coordinates of one point of the line
(x₂,y₂) are the coordinates of a second point of the line
First, determine two points of the line, for example, you can use the point (2,1) as (x₁,y₁) and the y-intercept with coordinates (0,-2) as (x₂,y₂)
Then, replace the coordinates of both points on the formula to calculate the slope:

The slope of the line on the graph is m=3/2