Answer based on context:

The Russo-Turkish War of 1735-1739 between Russia and the Ottoman Empire was caused by the Ottoman Empire's war with Persia and continuing raids by the Crimean Tatars. The war also represented Russia's continuing struggle for access to the Black Sea. In 1737, Austria joined the war on Russia's side, known in historiography as the Austro-Turkish War of 1737-1739.

How many years did the Russo-Turkish War last?
4