Determine the oxidation number of the specified element.
Cr in CrO4$\displaystyle ^2-$
well this is not math xD
but I know the answer, I think that you mean:
$\displaystyle
CrO_4^{ - 2}
$
to answer this problem you must know that, the oxidation number of oxigen y -2, then we have this equation:
$\displaystyle
Cr + 4 \cdot - 2 = - 2 \Leftrightarrow \boxed{Cr = 6}
$