what they want you to find is the slope and y-intercept, and put that into slope-int form, which is y=mx+b. the point that is located on the y intercept is (0, -3), which gives you your b, which is -3. the other point on the graph helps you find the rise/run, or slope, which is m=2/4. simplified is m=1/2. so, y=1/2 - 3
