Dividing Totals From Two Tables

I am writing a query where I need to get the total points the student received on assignments, this is in the grade table. I then need to divide that total from the total amount of points possible, located in the assignment table. At the end all I should see is the student_id and the percentage of the students grade.

Here is where I am so far

SELECT s.student_id, SUM(g.points)/SUM(a.points_possible) "GRADE"
FROM student s, grade g, assignment a
WHERE s.student_id = g.student_id
AND g.assignment_id = a.assignment_id
ORDER BY student_id;

However when I execute this is the error I get:

WHERE s.student_id = g.student_id
*
ERROR at line 3:

ORA-00937: not a single-group group function The asterisk's should be under the g I couldn't get it to line up.