We are given that helicopter is descending at 3.5 m/s and a package is dropped from helicopter.
Its speed relative to helicopter is 0, that means it is started with the same speed as helicopter that is 3.5 m/sec relative to ground.
And when it reaches ground, its speed is 13.4 m/sec relative to ground.
Packet speed relative to helicopter = packet speed - helicopter speed
= 13.4 - 3.5 = 9.9 m/sec