Answer :
If you call the gradient of S m:
Gradient of R if perpendicular to S = -1/m
Gradient of T if perpendicular to S = -1/m
-1/m=-1/m, so the lines are parallel
Gradient of R if perpendicular to S = -1/m
Gradient of T if perpendicular to S = -1/m
-1/m=-1/m, so the lines are parallel