$\Delta H=2 \times-46.0 \mathrm{\,kJ} \,\mathrm{mol}^{-1}$
Let $x$ be the bond enthalpy of $N-H$ bond then
$\Delta H=\sum$ Bond energies of products
$-\sum$ Bond energies of reactants
$2 \times-46=712+3 \times(436)-6 x$
$-92=2020-6 x$
$6 x=2020+92$
$\Rightarrow 6 x=2112$
$\Rightarrow x=+352\; k J / m o l$