Maybe I'm overlooking the simplicity of the question, but I keep coming up with answers separate to the given answer.

The angle between $\displaystyle \vec a$ and $\displaystyle \vec b$ is $\displaystyle 79 deg$.

Find $\displaystyle p$ if $\displaystyle \vec a = 6\vec i + 3 \vec j - 2 \vec k$ and $\displaystyle \vec b = -2 \vec i + p \vec j - 4\vec k$

Thanks in advance.