Let f be a function of two variables that has continuous partial derivatives and consider the points A(6, 9), B(12, 9), C(6, 11), and D(14, 15). The directional derivative of f at A in the direction of the vector AB is 6 and the directional derivative at A in the direction of AC is 5. Find the directional derivative of f at A in the direction of the vector AD. (Round your answer to two decimal places.)