Find the new equation of the conicoid 9x^2 + 16y^2 - 36z^2 - 36x - 72z = 144 when the coordinate system is changed into a new system with the same origin at (-2, 0, 1) and direction ratios same as the old system.
Show that the angle between the two lines in which the plane x - y + 2z = 0 intersects the cone x^2 + y^2 - 4z^2 + 6yz = 0 is tan inverse of (under-root 6 / 7) .