Liza hired a cleaning service to clean her house. To find C, the total cost of cleaning her house, she used the following formula: C = 7.5x + 25

What is the dependent variable in the formula?

The dependent variable in the formula is x.

No, C is the dependent variable.

since the cost (C) depends on the number of hours (x) that she worked

just like in y = mx + b, y is the dependent, and x is the independent variable.