How do I figure out the function of a graph?

[jon80]

I am trying to figure out the function of a graph that I sketched, and, I think I am getting it wrong. Please include the workings and sources of reference so I can look up similar problems in the future, please. What is the gradient anyway?

A copy of the sketched graph is available at https://skydrive.live.com/#cid=B712073B3513EB8E&id=B712073B3513EB8E%212177.
Please let me know when you are done with it so I can delete it.

Well, I think that the graph function is f(x) = 8 - 3x, however, I am not sure what trick can be used to find this out, because I based my function on:
When x is 0, y = 8; when y = 0, x = 3, taking the first co-ordinate to read the gradient, and, adding either +3 or -3 to find out the graph. Then I drew the graph using this formula on Graph 4.4 (www.padowan.dk), however, I still don't understand the mathematical basis for my "guess".

NOTE: It would be nice to allow attachments and mathematical symbols to be included within this forum, since we are talking math.

[stapel_eliz]

I am trying to figure out the function of a graph that I sketched, and, I think I am getting it wrong. Please include the workings and sources of reference so I can look up similar problems in the future, please. What is the gradient anyway?

The "gradient" is the British-English word for what Americans call "slope". Roughly speaking, it's related to the "angle" of a straight line from horizontal.

To learn how to use two points from a line to find the equation of that line, please try this lesson on straight-line equations.