Equipped creature has defender and ", : Other creatures you control gain trample and get +X/+X until end of turn, where X is this creature's power."
9/20/2014 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.
7/14/2017 If the equipped creature’s power is negative while resolving the activated ability it gains, X is considered to be 0. (This is a change from previous rulings.)