Let f be a function of two variables that has continuous partial derivatives and consider the points a(5, 6), b(9, 6), c(5, 13), and d(17, 15). 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 10. find the directional derivative of f at a in the direction of the vector ad. (round your answer to two decimal places.)