sunkyjay: I would like to ask a question cos i had an argument with someone that i benfica had won by 2-1, it going to be an extra time game. Just want to be sure

Away goals count double in the event of an aggregate draw. So if Benfica
had won 2 - 1, the aggregate score would have been 2 - 2. They will then
qualify for the semi-finals on superior away goals. The away goal rule has
been in effect for decades.

But i think away goals count only on 1st goal scored by an opponent.With Chelsea scoring an away goal first.

sunkyjay: But i think away goals count only on 1st goal scored by an opponent.With Chelsea scoring an away goal first.

The away goal rule comes in effect only when the aggregate score is
a draw. In that case the team that scores the most away goals wins. It
doesn't matter who scored first. If both teams score the same amount
of away goals, then the game will go to extra time and then penalties.

Below are a few examples

HOME AWAY
Benfica 0 Chelsea 1
Chelsea 1 Benfica 2 (Aggregate score: 2 - 2. Benfica wins on superior away goals)

Benfica 0 Chelsea 2
Chelsea 0 Benfica 2 (Aggregate score: 2 - 2. Extra time, then penalties if necessary)