find the slope between (5, 8) and (1, 7)

Formula for slope:  m  =  (y2 - y1) / (x2 - x1)

(x1, y1) = (5, 8)   --->   x1 = 5, y1 = 8

(x2, y2) = (1, 7)   --->   x2 = 1, y2 = 7

--->   m  =  (7 - 8) / (1 - 5)  =  -1/-4  =  1/4