Here's your answer :)
C7H6O3 (Salicylic acid) + C4H6O3 (Ethanoic anhydride) => C9H8O4 (Aspirin) + C2H4O2 (Acetic acid)
3.54 g / 138.12 g·C7H6O3 mol−1 = 0.0256 moles C7H6O3.
From eqn, 1 mole C7H6O3 reacts with 1 mole C4H6O3 so 0.0256 moles C4H6O3 reacted.
Mass acetic anhydride needed = 0.0256 mol * 102.0886 g mol-1 = 2.61 g (to 3 sig fig).
