y=mx+b is "Slope-Intercept Form" where "y" is the equation of the line, "m" is the slope of the line, and "b" is the y-intercept.
To find the y-intercept (or "b"), you should look for the point where the line crosses the y-axis. On this graph, the line crosses the y-axis when y=0, so b=0.
To find the slope (or "m"), you can use the trick "rise/run". Basically, find 2 points on the line that you know. You already have (-2,-1) and (2,1). To get from the lower point to the higher point, you must go UP 2 units. To get from there to the point you must go RIGHT 4 units. So you would have a slope of 2/4 which reduces to 1/2.
Therefore, you equation for the line would be y=1/2x