A.) We know that 300 chairs were rented each month in April and May. Also, that 600 chairs were rented each month from June to September.
B.) Equation-> April+May+June+July+August+September
All we need to do is plug in the values::
300+300+600+600+600+600
=3,000 chairs were rented during the 6 month period.
Hope this helps:)