In the Gauss - elimination for a solving system of linear algebraic equations, triangularization leads to

The eigen values of $$\left[ {\matrix{ 1 & 1 & 1 \cr 1 & 1 & 1 \cr 1 & 1 & 1 \cr } } \right]$$ are

Solve the system $$2x+3y+z=9,$$ $$4x+y=7,$$ $$x-3y-7z=6$$

Among the following, the pair of vectors orthogonal to each other is