2·x - 4·y + z = 3
x + 2·y + 2·z = 1
3·x - 2·y + a·z = 5
I + 2*II ; III + II
4·x + 5·z = 5
4·x + z·(a + 2) = 6
II - I
z·(a - 3) = 1 --> z = 1/(a - 3)
4·x + 5·(1/(a - 3)) = 5 --> x = 5/4 - 5/(4·a - 12)
(5/4 - 5/(4·(a - 3))) + 2·y + 2·(1/(a - 3)) = 1 --> y = a/(24 - 8·a)