What do you need at least three points to define?

In Geometry any two points uniquely define a line. In analytic geometry therefore, if you know any pair of points you can compute the equation of the line that passes through them.

If you’re drawing the graph of a line, I tend to recommend plotting three points, not because you need three points to determine your line, but as a check. If you compute one point incorrectly it’s obvious something’s wrong when you plot them. If you make two mistakes you have to be pretty unlucky for the three to line up nicely. It still should be obvious something’s wrong.
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Idon’t know but you’ll figure it out