What word describes the degree of desire for a good or a service ?

Dominic and stacy each have a stamp collection. dominic has six fewer stamps than three times the number of stamps stacy has. dominic and stacy decide to give all of their stamps to their younger sister, mary. if stacy had s stamps, which expression represents the number of stamps that mary received? a. (3s − 18)(3s − 6) b. 2s − 18 c. 4s − 6 d. 4s − 18