Complex sentence: Also, the airline began buying new 737s, ordering three Boeing 737-700s and becoming the launch customer for the Boeing 737-900 when it placed an order for ten of the jets in November 1997.
Split into simpler sentences: Also, the airline began buying new 737s, ordering three Boeing 737-700s and ten Boeing 737-900s. Alaska Airlines became the launch customer for the 737-900 when it placed an order for it in November 1997.