Answer:
y = 9 + 3x
Explanation:
$9 per month for a single user
$3 per month for additional users
Find monthly cost, y, where additional cost is represented by x
The cost for a single user = fixed cost (It remains constant)
The cost of the additional user = variable (It increases with the number of users)
The monthly cost increases with the number of additional users
The monthly cost = cost of single user + the cost of additional users
y = 9 + 3x