b)start with the first two
10X + Y + Z = 603
8X + 2Y + Z = 603
20X - 10Y - 2Z = -6
10X + Y + Z = 603
-(8X + 2Y + Z = 603)
-------------------------
2X - Y = 0
.
.
2X - Y = 0 (multiply this whole thing by 10)
20X - 10Y = 0
.
20X - 10Y - 2Z = -6
-(20X - 10Y = 0)
-2Z = -6
.
.