Firstly we need to count what is the difference between both altitudes:
44,000 - 9,000 = 35,000
If a plane descends at a rate of 2,500 feet per minute, then we have to count how many 2,500s fit in 35,000:
35,000 / 2,500 = 14
It means that the jet will reach its cruising altitude in 14 minutes.
