$p _{ i }= x _{ i } \times P _{ T }$

$P _{ i }=$ partial pressure of the $i ^{\text {th }}$ component

$x _{ i }=$ mole fraction of the $i ^{\text {th }}$ component

$p _{ T }=$ total pressure of mixture

$\Rightarrow 2 atm =\left(\frac{ n _{ H _{2}}}{ n _{ H _{2}}+ n _{ H _{ e }}+ n _{ O _{2}}}\right) \times p _{ T }$

$\Rightarrow p _{ T }=2 atm \times \frac{3}{1}=6 atm$