Solving a Supply and Demand Model with a Graph

The book's two equations are a demand curve and a supply curve written the usual way round for a graph, with price on the vertical axis. Demand is P = 8 minus half the quantity; supply is P = minus 0.4 plus a fifth of the quantity. Where the lines cross, the quantity buyers want equals the quantity sellers offer, and that is the equilibrium.

Drawn carefully, the crossing sits at a price of $2 and a quantity of 12, the same answer the algebra gives. The sliders change the demand curve's starting price and the steepness of supply, and the panel solves the two equations again each time, so the graph and the arithmetic stay in step.