Let f be a function of two variables that has continuous partial derivatives and consider the points A(1, 4), B(12, 4), C(1, 11), and D(13, 20). 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 15. Find the directional derivative of f at A in the direction of the vector AD. (Round your answer to two decimal places.)