What is the oxidation number of chromium in the ionic compound ammonium dichromate (nh4)2cr2o7?

The oxidation number of an element is the number of electrons that are gained or lost by the element to form a chemical bond. 
the net oxidation number of the ion is its charge.
compound (NH₄)₂Cr₂O₇ is made of cation - NH₄⁺ and anion - Cr₂O₇²⁻
oxidation number of ion - -2
(oxidation number of Cr x 2 Cr atoms) +(oxidation number of O x 7 O atoms )= -2
oxidation number of Cr = y
oxidation number of O = -2
(y x 2) + (-2 x 7) = -2
2y - 14 = -2
2y = 12
y  = + 6
therefore oxidation number of Cr is +6 

Ithink it’s carbon dioxide