First, I plotted both the lines on the graph, then I graphed the inequality.<u> </u><em>Please check out image.</em> Looking at the image, we can tell that the graph matches with Graph-C. Hence, Option C is correct.
You were going in the right direction but the problem was that you needed to subtract and not add. The formula should be y-y1 / x-x1 iirc. Hope this helped.