Which sign do i use??/

Herbert company gathered the following information on its mixed power costs and factory machine usage for the last four months: month power costs factory machine hours january $ 24,400 13,900 february $ 30,000 17,000 march $ 29,000 16,800 april $ 14,000 7,000 if it is estimated that 10,000 factory machine hours will be used in the month of may, what is the expected total power costs for may?