The value of X is determined as the activated ability resolves. If the equipped creature isn’t on the battlefield at that time, use its power the last time it was on the battlefield to determine the value of X. If that power was negative, other creatures you control will lose power and toughness. For example, if the equipped creature was 2/2 and a spell gave it -5/-5 in response to Dragon Throne of Tarkir’s activated ability, other creatures you control would get -3/-3 until end of turn.