3x+7x+z=1 ①
4x+10y+z=2006 ②
②-①得
x+3y=2005 ③
①乘以3得
9x+21y+3z=3 ④
②乘以2得
8x+20y+2z=4012 ⑤
④-⑤得
x+y+z=-4009
所以所求式子
=2005(x+y+z)/(x+3y)
=2005*(-4009)/2005
=-4009