Chapter 12 - Section 12.5 - Equations of Lines and Planes - 12.5 Exercises - Page 873: 53

Neither, $cos^{-1}|\dfrac{-1}{\sqrt {6}}| \approx 114.1 ^{\circ}$

$n_1=\lt1,2,-1\gt$ and $n_2=\lt 2,-2,1\gt$ Angle between two vectors: $\theta= cos^{-1}|\dfrac{a.b}{|a||b|}|$ $\theta= cos^{-1}|\dfrac{-3}{\sqrt {6} \sqrt {9}}|$ $\theta= cos^{-1}|\dfrac{-1}{\sqrt {6}}|$ $\theta \approx 114.1 ^{\circ}$ Hence, the two planes are neither parallel nor perpendicular.