First of all, you need to translate this sentence in order to find out which word to use to fill the gap. 'A comb and ___ are used to comb or brush your hair.' So, the word we need here is a brush. In Spanish, the word brush is el cepillo.
estar is to do with to be when it comes to people being somewhere, something being placed somewhere while Ser has to do with to be as in profession or to feel.
