Joonas buys a solar panel system for his house with an estimated annual yield of 4200 kWh, and pays for it with installation costs 6500 euros. Joonas lives south enough, so that the yield on the product system can be thought of as constant throughout the year. Joonas wants to reduce his electricity bill with an investment, which consists of a monthly fee of € 25 and electricity itself € 0.14 / kWh a) Mark the month since the system was installed with an x and form function S (x), which represents the savings in euros achieved by Joonas' investment. b) How much has Joonas saved in 3 years? c) Where to save the savings have amortized the purchase price of the system?