Math basics › Lines and systems
Slope
What it is
Rise over run: how much y changes for every 1 that x increases. m = (y₂ − y₁)/(x₂ − x₁).
Reading it
Positive climbs left to right, negative falls. Zero is flat. A vertical line has no slope at all — the run is zero and you cannot divide by it.
Keep the order
Whichever point you call ₁, use it first on the top and the bottom. Swapping one but not the other flips the sign.