Finding the equation of the line of best fit.

Hello

Basically I have a question where I have sketched two graphs for the first part. The second part of the question is telling me to find the equation of the line of best fit.

So I started with y=mx+b. From there I have to find m or the slope, which is:

slope=

__y2 - y1__

x2 - x1

But where do I get the two points on my graph? I have tried two random points, but I am getting weird answers for the slope.

After you get the slope you then plug in the slope and any point (X,Y) on the graph and thats how you get b or the y-intercept right?