That depends on whether resistance or armour applies first, assuming they don't stack additively (which would be stupid). Because there hasn't been a precedent of armour or physical damage reduction applying to elemental or chaos damage, there's no information on which is calculated first.
Essentially, you need to mitigate 16.67% of the incoming damage through armour before or after the remaining 70% elemental resistance. The armour you need for this is twice as much as the size of the hit.
With the pessimistic (realistic) assumption that armour applies before resistance, you would need 16k armour to mitigate a reasonably high 8k damage hit down to 2k including your resistance. If armour applies after resistance, you only need 4.8k armour for the same effect. If armour stacks additively with resistance, you need about 4.2k armour (because while the percentage of required reduced damage reduction to achieve this effect is much lower, the size of the hit considered would not yet have been reduced by resistance, thus lowering the effect of armour).