三元一次方程 急~~~~~please help me!!

x+y=1
y+z=15
z+x=4

(x+y) + (y+z) + (z+x) = 1+15+4

2(x+y+z) = 20

x+y+z = 10

x+y = 10 - z = 1 => z = 9
y+z= 10 - x = 15 => x = -5
z+x= = 10 - y = 4 => y = 6

(1)+(2)+(3): 2(x+y+z)=20
x+y+z=10______(4)

x=(4)-(2):(x+y+z)-(y+z)=10-15
x=-5

y=(4)-(3):(x+y+z)-(x+z)=10-4
y=6

z=(4)-(1):(x+y+z)-(y+z)=10-1
z=9

x+y=1----------1
y+z=15--------2
z+x=4-----------3
__________________
(1)-(3):
x+y-z-x=1-4
y-z=-3
y=z-3
________________
(1)-(2):
x+y-y-z=1-15
x-z=-14
x=z-14
____________
(2)+(3):
y+2z+x=19
z-3+2z+z-14=19
4z-17=19
4z=36
z=9
x=-5
y=6

三式相加--> 2(x+y+z)=20, x+y+z=10
z=9, x=-5, y=6

(x + y) - (y + z) = 1 - 15

x - z = -14

x + z = 4

So x = -5 and z = 9

y = 1 - x = 6