First, lets break up the equation into pieces.
The first variable is slope, in your case here, we have a slope of -3 or -3/1.
The other variable is the y-intercept, which is where place on the graph where you plot (0,X), in your case here we got (0,-4). That is the y-intercept.
to graph it you have to plot the y-intercept, and then identify the slope.
Here is an example.
