Final answer:
The monthly cost for x text messages can be found using the formula C = 7 + 0.35x.
Step-by-step explanation:
To find the monthly cost for x text messages, we need to add the cost of the plan ($7) to the cost of each text message ($0.35).
So the formula for the monthly cost is C = 7 + 0.35x, where C represents the monthly cost and x represents the number of text messages.
For example, if a student sends 100 text messages, the monthly cost would be
C = 7 + 0.35(100)
= 7 + 35
= $42.