Real Madrid must be able to overcome Salzburg first at the Santiago Bernabeu before playing away to Brest. It makes sense, although this is just a personal opinion, but I think Brest was very lucky in the last few games because they got slightly weaker opponents.
This match is very important for Real Madrid to determine their fate in the UCL, so they will play with high spirits to maintain their chances of qualifying through the Play-offs.
Salzburg can still be targeted by Real Madrid for 3 important points that will help them maintain their automatic qualifying position. If they win over Salzburg, then Real Madrid's points will be 12 which will be able to maintain their survival even though they have to fight through the playoff zone.
In order to qualify for the next round even though there are two more matches left, I feel that they can qualify for the last 16 even though Brest is an opponent that can be considered tough considering their achievements during their six matches in the Champions League.