11. For a certain function f: x), we know that f is proportional to x and that the constant of proportionality is 20. Find a formula for f. a) x} = 2a.: to x} = m C) !!!) 2 x + 21\".] d) x} = x20 12. A helicopter takes off from the roof of a building that is 210 feet above the ground. The altitude of the helicopter increases by 14D feet each minute. Use a formula to express the altitude of the helicopter as a function of time. a. Mm) = 210 14Um, altitude of the helicopter after at seconds, in feet b. Mm} = 21C! 140m, altitude of the helicopter after or minutes, in feet c. Mm) = 21 + l4m, altitude of the helicopter after H: minutes, in miles d. Mm} = 21C! + l4m, altitude of the helicopter after m minutes, in feet 13. A rental car agency charges $24.E per day and 3 cents per mile. Use a formula to express the cost of renting a car, in dollars, as a function of the number of days (d) you keep it and the number of miles (at) you drive