Final answer:
The longest river of Slovakia that forms the border with Hungary and merges with the Danube is the Tisza River.
Step-by-step explanation:
The longest river of Slovakia that forms the border with Hungary and merges with the Danube is the Tisza River. The Tisza begins in the Carpathian Mountains, flows through Slovakia, Hungary, and Serbia, and joins the Danube near the city of Novi Sad. The Tisza plays a significant role in the region's ecosystem and as a waterway for transportation and trade, similar to other major European rivers such as the Volga and the Danube.