I'm pretty sure you did it correctly
For this case we have the following situation:
The monthly cost of a gym in the city is $ 3. The initial registration is $ 25. Write a mathematical expression that models the problem.
So we have to define variables:
x: number of months
y: total cost
The mathematical expression that models the problem is:
The average of those two values is
(-11-4 +i+4i)/2 = -(15/2) +(5/2)i