How do variables you model real-world situations?

A relationship between two quantities is often expressed as an equation  in two variables. For instance, the equation:

is an equation in two variables x and y. Each order pair (a, b) will be a solution of this equation if and only if makes the equation to be true. That is, the ordered pair (1, -1) is a solution because:

In this way, the graph of an equation can  help you see relationships between real-life quantities. So, a real-world situation could be the life expectancy of a child, at birth, in the United States for selected years. So, if you are a researcher of a Research Institute you need to establish a relationship between the variable year and the variable life expectancy to know the life expectancy in each year.