What is the angle between the lines $2x =3y= -z$ and $6x =-y =-4z$ ?

$\frac\pi4$
None of the other choices
$\frac\pi3$
$\frac\pi6$
$0$
$\pi$

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The dot product is 0, so that the lines are right angles to each other. That is, if $t$ is the parameter, then

$(t)(t)+(\dfrac{2}{3}t)(-6t)+(-2t)(-\dfrac{6}{4} t) = 0$