This problem utilizes the Henderson Hasselbalch equation: pH = pKa + log [salt]/[acid] and they are asking you to find the [salt]/[acid], but actually the inverse.
pH = 4.2
pKa = -log 1.8x10-5 = 4.74
Substituting into HH equation: 4.2 = 4.74 + log [salt]/[acid]
log [salt]/[acid] = -0.5
[salt]/[acid] = 0.316
So, [acid]/[salt] or the ratio of acid to base = 1/0.316 = 3.16:1 mole ratio of CH3COOH to CH3COO-
If you want the mole ratio instead of molar ratio, just correct for the 750 ml, i.e 0.75 liters