Final answer:
The equation representing the monthly cost of a music-streaming service with additional users is y = 9 + 3X, where y is the total monthly cost and X is the number of additional users.
Step-by-step explanation:
The requested equation to show how the monthly cost, y, depends on the number of additional users, X, in the household for a music-streaming service is given by:
y = 9 + 3X
In this equation, the 9 represents the flat rate charged per month for a single user, and 3 represents the additional cost per month for each additional user. X is the number of additional users. Therefore, for each additional user added to the service, the cost increases by $3.