Find the valences of Z=26
Hint:
$$_{26}Fe\equiv1s^22s^22p^63s^23p^64s^23d^6\equiv [Ar]\ce{\underbrace{ ^ v }_{4s^2} \underbrace{ ^ v ,\;^  ,\;^  ,\;^  ,\;^   }_{3d^6} }$$
Think now what will be the configuration after losing electrons:

Spoiler:


  ! $$Fe^{2+}\to[Ar]\ce{\underbrace{ - }_{4s^2} \underbrace{ ^ v ,\;^  ,\;^  ,\;^  ,\;^ }_{3d^6}=[Ar]\underbrace{ ^ }_{4s^2} \underbrace{ ^  ,\;^  ,\;^  ,\;^  ,\;^   }_{3d^6} }$$
  
  ! $$Fe^{3+}\to[Ar]\ce{\underbrace{ - }_{4s^2} \underbrace{ ^  ,\;^  ,\;^  ,\;^  ,\;^   }_{3d^6} }
$$