What is the oxidation number of nitrogen in $N_3H$?

The oxidation number of $Cr$ in $K_2Cr_2O_7$ is

Which of the following involves the transfer of five electrons?

What is the oxidation number of $Li$ in $Li_2O$?

Assign the oxidation number to the underlined element in each of the following species:
$1$. $K_2\underline{Mn}O_4$
$2$. $Ca\underline{O}_2$
$3$. $Na\underline{B}H_4$
$4$. $KAl(\underline{S}O_4)_2 \cdot 12H_2O$

If an $M^{3+}$ ion loses $3e^-$,what will be its final oxidation number?