[填空题]
The plane $\Pi$ has the Cartesian equation x-y+p z=10 , where $ p \in \mathbb{R}$, p<0 .
The line L has the vector equation $\mathbf{r}=\left(\begin{array}{l}2 \\ 5 \\ 4\end{array}\right)+t\left(\begin{array}{l}1 \\ 1 \\ 1\end{array}\right)$,$ t \in \mathbb{R}$ .
The acute angle between $\Pi$ and L is $30^{\circ}$ . Find the value of p .
