Answer:
3
Explanation:
Northwest Woodworkers (NW) produced six times the quantity produced by Southwest Woodworking Guild (SW). That means the difference between the productions is 5 times (think NW produced 6 and SW produced 1 for example, overall, it's 6x but it's a difference factor of 5x).
Since we know the different in units was 15, we can say:
5x = 15, so x = 3
SW produced 3 bookshelves, while W produced 18. (6x as many, difference of 15).