Recall that slope intercept form is written exactly the way it's described: it gives you the slope of the line, then its y-intercept.
So first we can look at the graph, and see that for every 1 unit traveled to the right, the line travels 3 units down. Therefore our slope is -3.
Now we look for the y-intercept - we see the line intersects the y-axis at -3.
So our equation is:
y = -3x - 3