# What is the slope of a line that is perpendicular to (3,9) and (7, 15)?

Your line between $\left(3 , 9\right)$ and $\left(7 , 15\right)$ has slope $m$ given as:
$m = \frac{\Delta y}{\Delta x} = \frac{15 - 9}{7 - 3} = \frac{6}{4} = \frac{3}{2}$
The line perpendicular to this line will have slope ${m}_{1}$ equal to:
${m}_{1} = - \frac{1}{m} = - \frac{2}{3}$