To find the molecular formula from the empirical formula, you need to find a multiple (x) that will give you the molar mass of the compound which in the question is 54 g/mol.
If C₂H₃ is the empirical formula
molar mass of empirical formula = (12 × 2) + (1 × 3) g/mol
= 27 g/mol
let x = multiple
let molecular formula = C₂ₓ H₃ₓ
multiple = molecular mass ÷ empirical mass
= 54 g/mol ÷ 27 g/mol
= 2
If molecular formula = C₂ₓ H₃ₓ
then molecular formula = C₂₍₂₎H₃₍₂₎
= C₄H₆