Y=Mx+b. M refers to slope, which is rise over run. The rise (up and down movement) is -1 (because it goes down 1 to reach the next exact graph point). The run is 1 (because it moves 1 unit right to each the next exact point on the graph). The slope is -1/1, or just -1. B refers to the y intercept, which is the point where the graph hits the y axis. In this case. The y intercept is (0,3), or y=3. Your equation would then be y= -X+3