The demand curve illustrates the relationship between the price of a product and the quantity consumers are willing to buy. At lower prices, consumers may be willing to buy more but buy less as prices increase.
The supply curve shows the relationship between prices and quantities of products businesses are willing to produce. Businesses determine how much of their products to sell at a price that maximizes profits.
Linear equations may also model the relationship between investment and interest rates which show that as interest rates increase the general level of investment will decrease and increase as interest rates decrease.
You can also compare currency exchange rates with a linear equation to see how changes in the value of the Argentine peso affect the U.S. dollar.
Distances may also be modeled as a linear equation to show the relationship between changes in speed and time. A change in one these variables affects the distance traveled.