Answer:
Trisulfur nonaiodide
Explanation:
The name for a binary molecular compound has the form
Multiplying prefix+name of first element multiplying prefix+stem of second element element+ide (two words)
The multiplying prefixes for three and nine are tri and nona, respectively.
The stem of iodine is iod.
Put them together, and the name of S₃I₉ is
trisulfur nonaiodide.