slope

Let R be (x,y).
Since PQRS is a parallelogram, the horizontal distance between RS = horizontal distance between PQ,
so x - (-6) = 5 - ( -8)
x + 6 = 5 + 8 = 13
x = 13 - 6 = 7.
Similarly, the vertical distance between RS = vertical distance between PQ,
so y - 7 = - 2 - (- 4) = - 2 + 4 = 2
so y = 2 + 7 = 9
that means R is (7,9).
so slope of PR = (9 + 4)/(7 + 8) = 13/15.