Let f be a function of two variables that has continuous partial derivatives and consider the points A(3, 5), B(7, 5), C(3, 7), and D(11, 20). The directional derivative of f at A in the direction of the vector $\vec{AB}$ is 4 and the directional derivative at A in the direction of $\vec{AC}$ is 22. Find the directional derivative of f at A in the direction of the vector $\vec{AD}$. (Round the answer to two decimal places.)