How do you solve this question without using substitution?
Determine all ordered triples of integers (x, y, z) which satisfy the system
(x + y) (x + y + z) = 189
(y + z) (x + y + z) = 147
(z + x) (x + y + z) = 546
And if you're wondering, the solutions are (x, y, z) = (14, -5, 12) and (x, y, z) = (-14, 5, -12)