First of all, we need the slope of AB, which we can compute as
[tex] m_{AB} = \dfrac{\Delta y}{\Delta x} = \dfrac{17-3}{3-1}=\dfrac{14}{2} = 7 [/tex]
Now, if a line is perpendicular to a given line, their slopes follow this equation:
[tex] m_{AB}m_{CD} = -1 \iff m_{CD} = -\dfrac{1}{m_{AB}} [/tex]
So, the slope of CD is -1/7
