broom: sweep :: brush:_______
scrub
The relationship between "broom" and "sweep" is that a broom is a tool used to perform the action of "sweeping" surfaces. To find a similar relationship for "brush," we need to think of a verb that represents the action typically performed using a brush.
One such verb is "paint." A brush is commonly used to "paint" surfaces. Therefore, the analogy for the relationship between "brush" and this action is: brush:paint.