Let's look at this in a way that will get us that formula for crunching slopes from two points:
|Look at the as |
That is what gets us that familiar formula:
In math and science, we often use a Greek capital "D"... delta ( )... to represent a "change".
A Calculus notation you'll see a LOT ditches the Greek letter part and uses "d's"...
It's a simple thing from Algebra, but the slopes of lines are going to be REALLY important in Calculus.