πSid(σcredit<5 (course) (Natural Join)Enrolled) it will project all Sid's who enrolled all course having Credit <=4
πSid(Student) =It will list Sid's
Finally,
{All Sid's-All Sid's who enrolled all courses having Credit<=4}==All Sid's but Credit not<=4==students who enrolled all courses with credit greater than or equal to 5.