x+y+z = 66 (I)
y+3 = 2x (II)
x+y-z = 0 (III)
--------------------- (I) - (III)
2z = 66
z = 33
------------------------ (I) + (III)
2x + 2y = 66 |:2
x+y=33 (IV)
2x-y = 3 (II)
------------------------(II) + (IV)
3x = 36
x = 12
in (III)
x+y-z = 0 (III)
12 + y - 33 = 0
y = 33- 12
y = 21