Bråk is 'trouble', 'fight', 'row'. The verb is att bråka : 'to make a fuss', 'to quarrel' etc.
En bråkstake is 'a troublemaker'.
In mathematical terms bråk is 'fraction'. So "Räkna med bråk" is a pun, it means both 'do fractions' and 'count on trouble'.
In the dictionary I also found an English word that must be closely related: bråka lin is translated as 'brake flax'.