A gaseous mixture of H₂ and CO₂ contain 66 % CO₂ by mass of CO₂
this means if mixture 100g CO₂ =66g and H₂ =34g
Applying Dalton's law :
m mix = mH₂ + mCO₂
(n x M ) mix =(n x M ) H₂ + ( nx M )CO₂
100 x 2x = ( 34 x 2 ) + ( 66 x 44 )
x = 14.86
Hope it helps