This is the 4903rd most frequent French word.
sacrée
Sacrée: Sacred or holy.
In this sentence, 'sacrée' is used in the colloquial sense meaning 'quite' or 'considerable', emphasizing the extent of the surprise.
C'est une sacrée surprise !
It's quite a surprise!
Here, 'sacré' is used in its standard meaning which is 'sacred' or 'holy', describing something of religious or spiritual importance.
La cathédrale est un lieu sacré.
The cathedral is a sacred place.
In this context, 'sacrée' is used informally to mean 'tough' or 'challenging', stressing the difficulty of solving the equation.