Answer
a)$-2.78\times10^{-23}\,J/T$
b) $3.70\times10^{-23}\,J/T$
Work Step by Step
Recall that the z-component of the orbital magnetic dipole moment $\mu_{orb,z}=-m_{l}\mu_{B}$ where $\mu_{B}$(Bohr magneton)=$9.27\times10^{-24}\,J/T$.
a) When $m_{l}=3$, $\mu_{orb,z}=-3\times9.27\times10^{-24}\,J/T$
$=-2.78\times10^{-23}\,J/T$
b) When $m_{l}=-4$, $\mu_{orb,z}=-(-4)\times9.27\times10^{-24}\,J/T$
$=3.70\times10^{-23}\,J/T$
