Key Idea kohlrausch law of independent migration of ions can be used to calculate ${\Delta }_m^o$ for weak electrolyte such as acetic acid.
Given, ${\Delta }_{m\left(NaAC\right)}^o=91Sc{m}^{2}mo{l}^{-1}$
${\Delta }_{m\left(HCI\right)}^o=425.9Sc{m}^{2}mo{l}^{-1}$
${\Delta }_{m\left(NaCl\right)}^o=126.4Sc{m}^{2}mo{l}^{-1}$
Using Kohlrausch law, to find the molar conductivity of acetic acid.
${\Delta }_{m\left(HAC\right)}^o={\lambda }_{H^{+}}^o+{\lambda }_{A{C}^{-}}^o$
$={\lambda }_{H^{+}}^o+{\lambda }_{C{l}^{-}}^o+{\lambda }_{A{C}^{-}}^o+{\lambda }_{N{a}^{+}}^o-{\lambda }_{C{l}^{-}}^o-{\lambda }_{N{a}^{+}}^o$
$={\Delta }_{m\left(HCI\right)}^o+{\Delta }_{m\left(NaAC\right)}^o-{\Delta }_{m\left(NaCl\right)}^o$
$=(425.9+91.0-126.4)Sc{m}^{2}mo{l}^{-1}$
$=390.5Sc{m}^{2}mo{l}^{-1}$