Step-by-step explanation:
12x-3y+z＝-7 ‖-12x+3y
z＝-7-12x+3y
so you can replace z with (-7-12x+3y)
4y+3*(-7-12x+3y)＝-7
4y+(-21-36x+9y)＝-7
13y-21-36x＝-7 ‖+21
13y-36x＝14 ‖-13y
-36x＝14-13y ‖*(-1)
36x＝-14+13y ‖/3
12x＝-14/3+13/3y
so-6x＝+14/6-13/6y
(+14/6-13/6y)+2y-2*(-7-(-14/3+13/3y)+3y)＝12
(+14/6-13/6y)+2y+14-28/3+26/3y-6y＝12
(+14/6-13/6y)+2y+14-56/6+52/6y-6y＝12
7+2.5y＝12 ‖-7
2.5y＝5 ‖/2.5
y＝2
so
4y+3z＝-7
4*2+3z＝-7
8+3z＝-7 ‖-8
3z＝-15 ‖/3
z＝-5
12x-3*2+(-5)＝-7
12x-6-5＝-7
12x-11＝-7 ‖+11
12x＝4 ‖/12
x＝1/3
