So the rise/run is -4/1 (since the line goes down 4 units when it hits a line directly, and wherever the line hits (in this case unit 1 on the x-axis)). This will be your m. Slope intercept form is y=mx+b, therefore your equation will be y==4/1x+b. to find b, you need to look at where the line hits the y-axis. It hits it at point 4. therefore your final answer will be y=-4/1x+4