由(1)得x=3y/4 (3)
(3)代入(2)得 3y/4+2y-7z=0
z=11y/28
(x+3y+6z)/(x+5y+7z) 7后加了一个z
=(3y/4+3y+6*11y/28)/(3y/4+5y+7*11y/28)
=171/242