If Syr Faren leaves the battlefield before its triggered ability resolves, use its power as it last existed on the battlefield to determine the value of X.

The value of X is determined only as Syr Faren's triggered ability resolves. Once that happens, the value of X won't change later in the turn even if Syr Faren's power changes.

If Syr Faren's power is negative as its triggered ability resolves, X is considered to be 0.