Write the slope-intercept form of the equation for the line.

If I recall correctly, the slope-intercept form is y=mx+c or y=Mx+b

c is always the y-intercept of the graph, being the point where the line crosses the y axis. In this case:

c= -1

To get the slope, we can use the following equation:

m=(y2-y1)/(x2-x1)

All we need to do is substitute in two sets of coordinates. In this case we will use the coordinates highlighted with the bold points:

(1;2)
(-1;-4)

Now we will place the coordinates into the equation, making sure to correspond its x value to its y value:

m=(-4-2)/(-1-1)
m=3

Therefore the equation is y=3x-1