The given planes are7x + 5y + 6z + 30 = 0 ...(1)and 3x – y – 10z + 4 = 0 ...(2)These are neither parallel $\left[\because \frac{7}{3} \neq \frac{5}{-1} \neq \frac{6}{-10}\right]$ nor perpendicular [∵ (7) (3) + (5) (–1) + 6 (– 10) ≠ 0]If ‘θ’ be the angle between (1) and (2), then Hence $\theta = \cos^{-1}\left(\frac{2}{5}\right)$