Final answer:
The subject of the sentence is the person who goes bike riding on Sundays.
Step-by-step explanation:
The subject of the sentence 'Los domingos salgo a andar en bicicleta.' is the person who goes bike riding on Sundays. In Spanish, the subject is 'yo' (meaning 'I'). The verb 'salgo' indicates that the person is leaving or going out, while 'andar en bicicleta' means 'to ride a bike'.
Learn more about Sentence structure in Spanish