Let f be a function of two variables that has continuous partial derivatives and consider the points A(2, 5), B(10, 5), C(2, 10), and D(18, 17). The directional derivative of f at A in the direction of the vector AB is 2 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.)