Pack tactics — Whenever Targ Nar, Demon-Fang Gnoll attacks, if you attacked with creatures with total power 6 or greater this combat, attacking creatures get +1/+0 until end of turn.
{2}{R}{G}: Double Targ Nar's power and toughness until end of turn.
If a creature's power is less than 0 when it's doubled, instead that creature gets -X/-0, where X is how much less than 0 its power is. For example, if an effect has given a 2/2 creature -4/-0 so that it's a -2/2 creature, doubling its power and toughness gives it -2/+2, and it becomes a -4/4 creature.
If an effect instructs you to “double” a creature's power, that creature gets +X/+0, where X is its power as that effect begins to apply. The same is true for toughness.