How in the phrase <em>"</em><u><em>tu</em></u><em> cepillo"</em> is used a Possessive Adjective that owns to the personal pronoun <em>"tú"</em>, the possessive adjective must be for that pronoun too, and how the noun <em>"el cepillo"</em> is male and singular, the correct possessive pronoun is <u><em>"tuyo"</em></u>.