What are the dates of all the tests that have the result "Fail"?
select date_test_taken from student_tests_taken where test_result = "Fail"
How about that of all the tests that have the result "Pass"?
select date_test_taken from student_tests_taken where test_result = "Pass"
So what are the enrollment dates of all the tests that have the result "Pass"?
select t1.date_of_enrolment from student_course_enrolment as t1 join student_tests_taken as t2 on t1.registration_id = t2.registration_id where t2.test_result = "Pass"