Answer
$L_z = 3~\hbar$
Work Step by Step
In general, the values of $m_l$ can be $~~m_l = 0, \pm 1, \pm 2,...,\pm l$
If $l=3,$ then the largest possible value of $m_l$ is $~~m_l = 3$
We can write an expression for $L_z$:
$L_z = m_l~\hbar$
$L_z = 3~\hbar$
