The combined equation for a pair of planes is
S ≡ 2x2 - 6y2 -12z2 + 18yz + 2zx + xy = 0. If one of the planes is parallel to x + 2y - 2z = 5, then the acute angle between the planes S = 0 is
1
cos-1 \(\left(\frac{16}{21}\right)\)
2
\(\frac{\pi}{2}\)
3
\(\frac{2\pi}{3}\)
4
sin-1 \(\left(\frac{7}{15}\right)\)