An Equation expresses a relationship between (usually) two or more variables. For an equation to be a function there is only one output for each input.
In Economics Supply and Demand are each represented by an equation. They each have (quantity demanded) as a variable.
Each equation has 2 varianblea: Price () in dollars, and Quantity demanded .
A value for can be substituted into the equation.
When The Demand Curve tells us the Price
(remember order of operations)