Explanation:
12x+6y+72=-35
12x+6y=-35-72
12x+6y=-107 equation i
X+y=-10 equation ii
multiply by 6 on equation ii and subtract from equation i
12x+6y=-107
-6x-6y=60
6x=47
X=7.83333333
put the value of X on equation i
12×7.83333333+6y=-107
6y=-107-94
y=33.5
now
put the value of X and y on equation iii
7×7.83333333-5×33.5+6z=200
54.8333331-167.5+6z=200
-112.666667+6z=200
6z=200+112.666667
z=52.1111112