b) $\right)$ Find the directional derivative of $y\left(x,y,=\right)=xy^{3}+yz^{3}$ at the point $\left(2,-1,-3\right)$ in the direction of vector $2i$ $-3\bar{i} +42\bar{k} $ $\left(6M\right)$

$04//$ Let $fbea$ function of two variables that has continuous partial derivatives and consider the points $A\left(1,3\right),B\left(3,3\right)$ $C\left(l,$ $ca,7$ and $D\left(6,15\right).$ The directional derivative of j at $A$ in the direction of the vector $\vec{AB} $ is 3 and the directional derivative at $A$ in the direction of $\vec{AC} $ is $26$ Find the directional derivative of $y$ at $A$ in the direction of the vector $\vec{AD} $ $AD.$ $\left(S$ $narky\right)$ $y$

Find the directional derivative of $O=x^{2}-2y^{2}+4z^{2}$ at $\left(1,1,-1\right)$ in the direction of $2l+-X$ $8$ $=$