how do u solve a system of linear equations in three variables?

Question

for example this problem
Ex.1 2x-2y+z=3
5y-z=-31
x+3y+2z=-21

Answer 1

http://www.mathwarehouse.com/algebra/planes/systems/three-variable-equations.php

Answer 2

y=3x+4

Answer 3

81x2-18x+1