Slope: Parallel, perpendicular, or neither

Determine whether the line passing through the first pair of points is parallel, perpendicular, or neither to the line passing through the second pairs of points.

(a,b) and (b,a); (c,d) and (d,c)

They're parallel, both slopes are -1

the first slope is given by:

(b - a) / (a - b)
=
-1(a - b) / (a - b)
=
-1

for the second it's the same, but swap out a and b for c and d.