Demand function is the most basic knowledge of economics. The demand function describes the relationship between output and price. The general expression of linear demand function is Qd=a-bP, where a and b are constants. This expression means that the price of the product will affect the demand of the product. The higher the product price, the less the product demand.
It is not that it is meaningless to subtract quantity from price, but that the demand function reflects the mathematical relationship between price and output.
Do you know that distance = speed * time? When the speed is 1, the image of distance and time is Y = X, why x seconds =y meters? ......
Game theory is a relatively late chapter in microeconomics ... I suggest you read the previous content first. ...