Complex sentence: At the invitation of the Iranian government, Zakka traveled to Tehran on September 15,2015 to participate in a conference on entrepreneurship and disappeared on September 18 on his way to the airport to leave the country.
Split into simpler sentences: At the invitation of the Iranian government, Zakka traveled to Tehran on September 15,2015 to participate in a conference on entrepreneurship. On September 18, he disappeared while on the way to the airport to leave the country.