Complex sentence: On October 29, William C. Durant joined with members of the Rockefeller family and other financial giants to buy large quantities of stocks to demonstrate to the public their confidence in the market, but their efforts failed to stop the large decline in prices.
Split into simpler sentences: On October 29, William C. Durant joined with members of the Rockefeller family and other financial giants to buy large quantities of stocks to demonstrate to the public their confidence in the market. But, their efforts failed to stop the large decline in prices.