The best way to get a good idea of this is to graph the line. Here is the graph.
We know that the slope is the rise over the run which is -2/2 or -1
we can also see from the graph that the y intercept is located at -2
The equation for a line is y=mx+b where m equals the slope and b equals the y intercept.
Plug in the values for this line and we get y=-x-2