The set of points $(x,y,z)$ that satisfy \[2x = 3y = -z\]is ...

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Correct answer: 90^\circ Solution: For the first line, let $t = 2x = 3y = -z.$ Then \[\begin{pmatrix} x \\ y \\ z \end{pmatrix} = \begin{pmatrix} t/2 \\ t/3 \\ -t \end{pmatrix} = \frac{t}{6} \begin{pmatrix} 3 \\ 2 \\ -6 \end{pmatrix}.\]Thus, the direction vector of the first line is $\begin{pmatrix} 3 \\ 2 \\ -6 \end{pmatrix}.$ For the second line, let $t = 6x = -y = -4z.$ Then \[\begin{pmatrix} x \\ y \\ z \end{pmatrix} = \begin{pmatrix} t/6 \\ -t \\ -t/4 \end{pmatrix} = \frac{t}{12} \begin{pmatrix} 2 \\ -12 \\ -3 \end{pmatrix}.\]Thus, the direction vector of the first line is $\begin{pmatrix} 2 \\ -12 \\ -3 \end{pmatrix}.$ Note that \[\begin{pmatrix} 3 \\ 2 \\ -6 \end{pmatrix} \cdot \begin{pmatrix} 2 \\ -12 \\ -3 \end{pmatrix} = 0.\]Hence, the angle between the lines is $\boxed{90^\circ}.$