Solve the differential equation using the method of variation of parameters. 
Definitions:
Demand
The quantity of a product or service that consumers are willing and able to purchase at various prices.
Sales
Transactions in which goods or services are exchanged for money.
Supply
The total amount of a specific good or service that is available to consumers, often influenced by price.
Equilibrium Price
The price at which the quantity of a good or service demanded by consumers equals the quantity supplied by producers, resulting in market balance.
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