Answer
$4.5Kg$
Work Step by Step
We can find the mass of the hanging block as follows:
$\Sigma F_x=T-m_2gsin\theta=0$
$\implies T=m_2gsin\theta$
$\implies m_1g=m_2gsin\theta$ (where $m_1$ is the mass of the hanging block and $m_2$ is the mass of the second block)
$\implies m_1=m_2sin\theta$
We plug in the known values to obtain:
$m_1=(6.7)sin42^{\circ}$
$m_1=4.5Kg$
