Answer
$-r = t-s$
Work Step by Step
We use the equation for slope:
$m = \frac{y_2-y_1}{x_2-x_1}$
We set the two slopes equal to each other, for they are parallel:
$ \frac{v}{-r} = \frac{v}{t-s} \\ \frac{1}{-r} = \frac{1}{t-s} \\ -r = t-s$
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