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. (This is not the value of X you chose when you cast the spell)

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 5/5 creature -7/-0 so that it’s a -2/5 creature, doubling its power makes it a -4/5 creature.

Exponential Growth doubles a creature’s power sequentially, not all at the same time. For example, if X is 3 and you target a 2/2 creature, you will double its power once to 4, again to 8, and a third time to 16, making it a 16/2 creature until end of turn.