Answer:
20x+90
Explanation:
Monthly fee is a hint to you, that its a fee that happens monthly so we know that, that specific value costs will have a variable containing the number of months you attend the gym.
The one time fee of 90$ means it happens once you apply and it will no longer apply after registration.
Our equation would be Total cost: 20x + 90.
You can give total cost like f(x) if you are comfortable with that.
Hope this helps.