1. Tu sabes el precio del vestido?
This sentence means - Do you know what the price of this dress is? Both saber and conocer are translated the same into English (=to know), but saber is to know with the meaning of actual knowledge (I know math well), whereas conocer means that you know someone or someone's work (I know Maria well).
2. Usted conoce a mi hermana Maria, no?
Here you have to use conoce because the question is asking - Do you know my sister Maria? Just apply the appropriate suffix for usted (third person singular), and there you will have your correct answer.
3. Nosotros sabemos hacer windsurf.
This sentence means - We know how to windsurf. Obviously you would use the verb 'saber' here because you want to show that you have the knowledge of doing something, not that you know a particular person.
4. Yo se donde esta la corbata azul.
This sentence means - I know where the blue tie is. Naturally, you would use the verb 'saber' here as well to denote that you know the location of that particular thing you are looking for, rather than a person or their work.
5. Ellas conocen los libros de Carlos Fuentes?
The sentence means - Do they know novels by Carlos Fuentes? Here, you would use the verb 'conocer' meaning that they are familiar with this author's works, rather than 'saber' which wouldn't mean anything in this context.
en la clase de arte, yo dibujo
