Let f be a function of two variables that has continuous partial derivatives and consider the points A(8, 9), B(9, 9), C(8, 12), and D(14, 17). The directional derivative of f at A in the direction of the vector AB is 3 and the directional derivative at A in the direction of AC is 12. Find the directional derivative of f at A in the direction of the vector AD. (Round your answer to two decimal places.)