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.
The Santiago Bernabeu is a atmospheric ground where Real Madrid dominate and thrash oppositions, they don't welcome threat and easily defeats oppositions. We have weaker teams in UEFA Champions league and soon enough, they will eliminated from UCL. There are main elite teams in the UCL and competitive race is really tough. This is not supposed to be a debate for Real Madrid because we're acquainted with the historic records of Los Blancos, they're enchanced with potentials and winning games is necessary. Carlo Ancelotti knows what to do and comprehending matches is no difficulty for him, he's experienced and capable of hitting tougher goals with his team.