SQL joins/update

Hi I am having a great deal of trouble with this join command and my professor's clarity is killing me.

table book_copy:
(CO, Book_ID, Category, DATE_DUE, Cust_IDN, NDATE_OUT, RDATE_OUT, Cust_IDR)

...where Cust_IDN is when a customer checks out a normal copy and Cust_IDR is when a customer checks out a reserve copy.

table check_out:
(Category, Max_Days)
this table shows the relationship of how long the customer can check out the book eg.

Category Max_Days
________ _________
faculty 70
employee 50
student 40
associate 20

table customer:
(Cust_ID, Category)

category as in faculty, employee, student, associate

What he wants is for us to make a join statement that updates
date_due according to the ndate_out and rdate_out (when it was checked out).

what I have so far is:

update copyBC
set date_due = (select ndate_out + max_days
from check_out_rule, customer
where copyBC.CIDN = customer.CID and check_out_rule.category = customer.category);

but it is not working... any help would be great thanks.