"how could you know the primary key of that table has to be studentnumber plus course? thank you."
to get unique values for the primary key
Because more than one student will take each course.
Remember, you don't just look at the data SAMPLE you have, you must also consider the meaning of the data and the scenario where it will be used.
"the course id the pK. studentnumber is the foreign key. now, does the table fit for the 3NF?"
no, you need the joint key, no it is not 3nf
BECAUSE, you may also have a course table, remember. And now think of those non-mathematicians who take a course called maths - will it be the exact same course as that taken by the mathematicians? What's the PK for the course table then? How should you now modify the table which currently has course, credit, and studentnumber? Even assuming there is only one course called maths, and all course names are unique, it needs modifying.