Question 12407


  x + 4y + z = 12.........(A)
  y - 3z = -7 ............(B)
  z = 3 ..................(C)

   subst (C) in  B
  y= -7+3(3)
   =2      ................(D)



subt. (D)&(C) in (A)
 x+4(2)+3 = 12
          =12-8-3
          =1

after you can check that your X ,Y & Z are correct by plugging them back in the equation (A):
   
         1 + 8 + 3 = 12
               0   = 0