Let f be a function of two variables that has continuous partial derivatives and consider the points A(9, 5), B(11, 5), C(9, 7), and D(17, 11). The directional derivative of f at A in the direction of the vector AB is 5 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.)