Answer
a) ${ 17}\;\hat k$
b) $0$
Work Step by Step
a) Since we need the product of $\vec A\times \vec B$, so we need to find the angle from $\vec A$ to $\vec B$, as seen in the figure below.
$$\vec A\times \vec B=AB\sin\theta$$
where $\theta=180^\circ -135^\circ=\bf 45^\circ$
Hence,
$$\vec A\times \vec B=(6)(4)\sin45^\circ\;\hat k$$
$$\vec A\times \vec B=\color{red}{\bf 17}\;\hat k$$
where $\hat k$ is the $z$-direction out of the page toward you according to the right-hand rule.
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b)
$$\vec C\times \vec D=CD\sin\theta$$
where $\theta=180^\circ$
$$\vec C\times \vec D=(6)(4)\sin180^\circ=\color{red}{\bf 0}$$
