Answer 1

select avg(price)
from apartments
where status = 'Not rented'
and ...

https://code.sololearn.com/cYsGY4y3pD71

Answer 2

select price avg(price) won't work, try this instead

select avg(price) from apartments and status = 'Not rented'

Answer 3

select * from Apartments
where price > (select avg(price) from Apartments) and status = 'Not rented'