Answer
$m=\dfrac{y_2-y_1}{x_2-x_1}$
Work Step by Step
Divide both sides by $x_2-x_1$ to find:
$\dfrac{y_2-y_1}{x_2-x_1}=\dfrac{m(x_2-x_1)}{x_2-x_1}
\\\dfrac{y_2-y_1}{x_2-x_1}=m
\\m=\dfrac{y_2-y_1}{x_2-x_1}$
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