What numbers are needed for: _*(_+_)/_=6

Leslie started last week with $1200 in her checking account. during the week, she wrote the checks below.

Two airplanes start at the same place and travel in opposite directions,one at 395 miles per hour and the other at 422 miles per hour. how many hours will it take for the planes to be 2451 miles apart?

Which property was used to simplify the expression? 3c+9 +4c=3c+4c+9