Let f be a function of two variables that has continuous partial derivatives and consider the points
A (4, 9),
B (7, 9),
C (4, 13), and D(16, 18).
The directional derivative of f at A in the direction of the vector AB is 8 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.)