这两到3元1次方程组怎么做呀

x + y = 2a .(1)
y + z = 2b .(2)
z + x = 2c .(3)
由(1)得到：y = 2a - x
由(2)得到：z = 2b - y = 2b - (2a - x) = 2(b - a) + x 代入(3)得到：
2x = 2c - 2(b - a)
x = a - b + c
y = a + b - c
z = -a + b + c
.
2x - y - z = 3 .(1)
4x + 2y - z = 15 .(2)
44x + 3y = 18 .(3)
(2) - (1)得到：2x + 3y = 12 .3y = 12 - 2x 代入(3)得到：
44x + (12 - 2x) = 18
42x = 6
x = 1/7
y = 82/21
z = -139/21