First as you would have to work out the g/s`s for homewards bound and outwards flights.Failed heading home!
Next you want to see how much fuel you burn against distance travelled.
For this take the g/s and divide it into the fuel flow.I got failed eng 570/143= 3.99kg/nm(heading home 210trk) norm i got 3.56kg/mn Add these (7.55kg/hr)to find consumption flying out on two and back on one!
From first part/leg of the flight you would have burned 2100 for 2.5hr flight so 7000-2100=4900kg left! you normally would be given a diversion and reserve to subtract which dose`nt seem to be here so i took a 45mins reserve to arrive back with?
This now gives you 4900-630=4270kg
Finally divide this by first part you worked out(7.55) so 4270/7.55=565nm for your answer.
I have to stress its a year since i did any of this,but it gives you a start.They didnt really cover it in the atpl brush up.But you never know
Sorry should have said they did cover it but its hardly ever used in the exams!!