Complex sentence: On 7 January 2015, two French Muslim extremists attacked the Paris headquarters of "Charlie Hebdo" and killed thirteen people, and on 9 January, a third terrorist killed four hostages during an attack at a Jewish grocery store at "Porte de Vincennes".
Split into simpler sentences: On 7 January 2015, two French Muslim extremists attacked the Paris headquarters of "Charlie Hebdo" and killed thirteen people. On 9 January, a third terrorist killed four hostages during an attack at a Jewish grocery store at "Porte de Vincennes".