As the initial momentum of the bomb was zero, therefore after explosion the two parts should possess numerically equal momentum i,e $m_A v_A=m_B V_B$
$\Rightarrow 4v_A= 8\times 6$
$\Rightarrow v_A=12ms^{-1}$
Kinetic energy of the other mass
$A=\frac{1}{2}m_A v^2_A=\frac{1}{2}\times 4\times (12)^2$
=288 J