select count(distinct t1.author_id)
from answer_tb
join issue_tb
on answer_tb.issue_id=issue_tb.issue_id
join 
    (select distinct author_id
    from answer_tb
    join issue_tb
    on answer_tb.issue_id=issue_tb.issue_id
    where issue_type='Education'
    ) t1
on answer_tb.author_id=t1.author_id
where issue_type='Career';