Collect enough data about the caf$$ coffee day's sales $($at least ten years' quarterly data or five years' monthly data$)$ and use that as the dependent variable. For independent variables, the possibilities are the corresponding data on price, some economic index $($as proxy for purchasing power of customers$),$ marketing budget, competitor' price, competitor's marketing budget and$/$or any other data that you think are relevant. The most important consideration here is whether that data can be collected. You must have data on at least five independent variables. Your final model should have at least three independent variables.