I need help with a subquery!

Change this into a subquery
SELECT a.slname as LastName, a.sfname as FirstName, b.cname as CoureTitle, c.grade as Grade
FROM banerjee.student a, banerjee.course b, banerjee.enrollment c, banerjee.course_section d
WHERE c.grade is NOT NULL
AND a.sid = c.sid
AND c.csecid = d.csecid
AND d.cid = b.cid;