This question is asking for slope-intercept form which is y=mx+b, where m is the slope and b is the y-intercept. First, find the slope; this can be done in two ways, counting change in y and x then dividing, or using the formula . Either way for this graph the slope comes out as -1/3. Next, find the y-intercept, this is where the line intersects with the y-axis. In this case, the line touches the axis at 5, therefore 5 is the value of b.
