If you know two points on a line you can solve for slope, m (some people say rise over run, or change in y over change in x). In the attached picture, it's the top equation in blue.
Then you can plug that into the point-slope formula (middle equation, in red), which if needed can be algebraically changed to look like the slope-intercept formula (third equation, in yellow).
But to answer your question, use the point-slope formula.
