Alright, you're teaching about the rule for slopes of perpendicular lines in Algebra or Geometry.
Here are some of the instructional strategies or approaches you may have used...
(1) State the theorem without explanation followed by 3-4 demo examples of how it's used
(2) Motivate the theorem using the lines y = (3/4)x and y = (-4/3)x, choosing the points (4,3) and (-3,4) to demonstrate why these lines are perpendicular
(3) A more abstract approach using the following diagram