Question 1101425
It differs based on the TYPE of graph/function.
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If the graph is that of a straight line, then 2 points are all you need.  The letter m is often used to denote the slope of a straight line:

   m = (y2-y1)/(x2-x1)    

   where (x1,y1) and (x2,y2) are the two points on the line.
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For higher order lines (curves), you need to take the derivative of the function represented by the graph.  The derivative is the slope of a tangent line to the curve at a given point.
     
For example,  if  y = x^2  then  dy/dx = 2x.   So the slope of a line tangent to x^2  at, say, x=3, is 2*3=6.  The slope of the tangent line at, say, x=6, is 2*6=12  (in contrast to the straight line case, notice how the slope changes).