To calculate damage, this is how you do it:
Damage = ((((2 Level / 5 + 2) AttackStat AttackPower / DefenseStat) / 50) + 2) STAB Weakness/Resistance RandomNumber / 100
So, in your case, since you are using power whip, this is the calculation:
Damage = ((((2 Level / 5 + 2) 312 120/ DefenseStat) / 50) + 2) STAB Weakness/Resistance RandomNumber / 100
Random number is a random integer number between 85 and 100, called roll damage. Since you haven't told me the defending Pokemon, or your Ferrothorn's level, this is as much as I can tell you.
Hope this helped!