A Local Gym Has Discovered That Their Demand for Personal

Short Answer

A local gym has discovered that their demand for personal trainers (measured in hours) is related not only to their own advertising expenditures in the prior month, but also to the demand for doughnuts and swim gear in the prior month at neighboring stores. The gym has developed the following regression model to forecast demand for personal trainers:
Demand = 185 + (0.15∗advertising expenditures) - (0.05∗doughnuts) + (0.23∗swim gear sales)
What is the forecast for October, given advertising expenditures of 1000, doughnut sales of 2450, and swim gear sales of 782 in September?