#### Answer

For the molecular formula, count the atoms in the condensed structural formula given: $\mathrm{C}_{4} \mathrm{H}_{6} \mathrm{O}_{2} $.
The empirical formula has the same ratio but with dividing by a common denominator to get the smallest possible integers: $\mathrm{C}_{2} \mathrm{H}_{3} \mathrm{O}_{2} .$
For the structural formula, imagine the formula of butane and replace the $-C H_{3}$ groups at the two ends of the butane structure with $\mathrm{- COOH}$ groups.
For the line-angle formula, draw a line to represent $\mathrm{H}_{2} \mathrm{C}-\mathrm{CH}_{2}$ and at an angle at each end of the line put a $\mathrm{- COOH}$ group.