Let f be a function of two variables that has continuous partial derivatives and consider the points A(9, 2), B(10, 2), C(9, 8), and D(15, 10). The directional derivative of f at A in the direction of the vector AB is 4 and the directional derivative at A in the direction of AC is 14. Find the directional derivative of f at A in the direction of the vector AD. (Round your answer to two decimal places.)