Question 59901
The rate of change is the slope or the "slant" of the line.
(y1-y2)/x1-x2) [The slope formula]
Given points (1,2) and (3,4)
Slope=m=(2-4)/(1-3) [Plug-in the values for y1, y2, x1, and x2; and solve]
Slope=m=(2-4)/(1-3) =-2/-2=1

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The sign (positive or negative) of the slope determines whether it is a postive or negative slope:
Example:  m = -2 is a negative slope
{{{ graph( 300, 200, -6, 5, -10, 10, 2x-2 ) }}} 
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Example: m=+2 is a positive slope
{{{ graph( 300, 200, -6, 5, -10, 10, -2x-2) }}}