Using the slope-intercept form of y = mx + b, just plug in for the values given by the graph.
Your Y-intercept (0, y), you plug in the y value as B. M is where you put your slope.
So for the second one (top right), you have a Y-int of -2 or (0, -2) and a slope/m of 1/1 = 1. So your equation would be y = 1x - 2 or y = x - 2.
