Problem Solving - Unclear problem

From Math Power 10, Section 3.9, Factoring topic, page 130, question# 21

Question: 
Factor if possible 9x^2 - 15x - 4, the answer in the text book is not possible  

To my opinion, the question is unclear when students are asked to find the solution with integers only. Actually, we can factor this expression, as long as the determinant b^2 - 4ac > 0, the corresponding equation has 2 distinct real roots, let call them x1 and x2 then we can factor the expression as   a(x - x1)(x - x2).

In this case the determinant is (-15)^2 - 4(9)(-4) > 0, so the corresponding equation has two different real roots, they are irrational numbers, so we can factor this expression as 9(x - x1)(x - x2)

This section, to my opinion, gives students a misleading concept about factoring a quadratic expressions since the irrational roots of quadratic expressions are ignored in the process of factoring.