Descent is planned from 7500 ft AMSL so as to arrive at 1000 ft AMSL 6 NM from a VORTAC.

With a GS of 156 kts and a rate of descent of 800 ft/min. The distance from the VORTAC when descent is started is:

You must descend 6500ft (7500ft to 1000ft) at 800ft/min … so in terms of TIME, it will take: 6500/800 = 8.125 mins

Your G/S is 156kts … distance travelled = 156 x 8.125/60 = 21.1nm

So you must start the descent 21.1 + 6 = 27.1nm from the VORTAC