We will rewrite the formula as an equation.
2K + 2Cr + 7O = 0
The zero comes from there being no overall charge on the compound.
Each oxidation number is represented by the element's symbol.
Since this compound is not a peroxide or superoxide, the oxidation value for O is -2
K being in group 1 has a +1 charge and therefore has an oxidation number of K is +1
Now, by putting known values in we have:
2(+1) + 2(Cr) + 7(-2) = 0
2 + 2Cr -14 = 0
2Cr = +12
Cr = +6.
