x+y-z=70 *1
15x+5y+10z=70----------3x+y+2z=14 *2
*2-*1 :2x+z=-56----------z=-(2x+56)
x+y-z=x+y-[-(2x+56)]=3x+y+56=70---------3x+y=14 *3
3x+y+2z=3x+y+2[-(2x+56)]=14-----------x+y=126 *4
*3-*4 :2x=-112----------x=-56
z=56
y=182看错题了吧？