Complex sentence: On March 3,1821, Thomas L. Jennings became the first African-American to be granted a United States patent, for his cleaning process called "dry scouring," which was the precursor to dry cleaning.
Split into simpler sentences: On March 3,1821, Thomas L. Jennings became the first African-American to be granted a United States patent. The patent was for his invention of a cleaning process called "dry scouring," which was the precursor to dry cleaning.