2z - 31 = -9z + 24 can anyone tell me how to know if we must subtract or add? like we must write it as this when we solve it 2z - (- 9z) or 2z + -9z

Derek purchased a new gym membership. it costs him seven dollars a month plus two dollars for every time that he visits the gym. he wants to spend less than 25 dollars a month to visit the gym. write an inequality that models the situation using the variable v for the number of visits per month. also find the maximum number of visits he can make to the gym and still keep his cost below 25 dollars a month.