Ответ:

  1. \(2x=840\), \(x=840:2=420\).
  2. \(x:19=19\), \(x=19\cdot19=361\).
  3. \(x:3-18=12\); \(x:3=30\); \(x=90\).
  4. \(120:(25-x)=10\); \(25-x=12\); \(x=13\).
  5. \(5x+80=690\); \(5x=610\); \(x=122\).
  6. \(708-x+130=500\); \(838-x=500\); \(x=338\).