Calculus with Applications (10th Edition)

$m=0$
Through $(1,5)$ and $(-2,5)$ The slope of a line is given by $m=\dfrac{y_{2}-y_{1}}{x_{2}-x_{1}}$, where $(x_{1},y_{1})$ and $(x_{2},y_{2})$ are points through which the line passes. Substitute the points given into the formula and evaluate: $m=\dfrac{5-5}{-2-1}=\dfrac{0}{-3}=0$