I remember it seems that you multiplied the final exam results of each subject by the credits of that subject, and then added them up and divided them by the total credits you took this semester, and you got it, got it? take for example

90 points and 5.5 credits for advanced mathematics

75 points in English, 4.5 credits.

80 points in chemistry, 2.5 credits

etc

(90x 5.5+75x 4.5+80x 2.5)/(5.5+4.5+2.5)= 1

It's just that you have to count all subjects, but physical education class and elective courses don't seem to count.