There are several ways to do this. I'll show you two methods.
1) Pick two points on the line and use the slope formula. Look for two points that are easy to read. It is best if the points are on grid line intersections. For example, you can see points (-4, -1) and (0, -2) are easy to read. Now we use the slope formula.
slope = m = (y2 - y1)/(x2 - x1)
Call one point (x1, y1), and call the other point (x2, y2). Plug in the x1, x2, y1, y2 values in the formula and simplify the fraction.
Let's call point (-4, -1) point (x1, y1). Then x1 = -4, and y1 = -1. Let's call point (0, -2) point (x2, y2). Then x2 = 0, and y2 = -2.
2) Pick two points on the graph and use rise over run. The slope is equal to the rise divided by the run. Run is how much you go up or down. Rise is how much you go right or left.
Pick two easy to read points. We can use the same points we used above, (-4, -1) and (-0, -2). Start at point (0, -2). How far up or down do you need to go to get to point (-4, -1)? Answer: 1 unit up, or +1. The rise is +1. Now that we went up 1, how far do you go left or right top go to point (-4, -1)? Answer: 4 units to the left. Going left is negative, so the run is -4. Slope = rise/run = +1/-4 = -1/4
As you can see we got the same slope using both methods.
