Complex sentence: On May 14,2000, supporters claimed that approximately 750,000 mothers and others gathered on the National Mall in Washington, DC to demand sensible gun laws, and that an additional 150,000 to 200,000 people marched in support events across the country.
Split into simpler sentences: On May 14,2000, approximately 750,000 mothers and others gathered on the National Mall in Washington, DC to demand sensible gun laws. An additional 150,000 to 200,000 people marched in support events across the country.