Here's one way to do it:
(it helps if you write down what is happening to the relations after every expression)
Select courses done by student no. 2310
$Temp_1 \leftarrow \Pi_{Cno}[\sigma_{Student\_no=2310}(COMPLETED)]$
Rename the relation to student_courses and its attribute to Pre_Cno to make it ready for division
$\rho_{student\_courses(Pre\_Cno)}(Temp_1)$
All courses paired with the courses done by student 2310
$Temp_2 \leftarrow COURSES \times student\_courses$
Select Cno's from COURSES
$Temp_3 \leftarrow \Pi_{Cno}(COURSES)$
Rename it to misc_courses and its attribute to Cno1 to distinguish it from Cno
$\rho_{misc\_courses(Cno1)}(Temp_3)$
Final dividend
$Temp_4 \leftarrow \sigma_{misc\_courses.Cno1=Temp_2.Cno}(misc\_courses \times Temp_2)$
This will give us all the courses (twice, once as Cno1 and the other as Cno) with their names paired with each of the courses done by student no. 2310 (as Pre_cno) so that dividing this by the PRE_REQ relation will give you the name and course no. (as Cno1) of the required courses
$Temp_4 \div PRE\_REQ$
(Have a look at the wiki page if you don't know what the division operator does)