X^3– is isoelectronic with Ar. It has electrons and neutrons in equal number. The mass number of X is

${\mathrm{X}}^{3-}=\left[\mathrm{Ar}\right] \mathrm{which} \mathrm{contain} 18 \mathrm{electrons}$

so, element ‘X’ has 15 protons

$\therefore$ Mass number = p + n = 15 + 18 = 33