Chapter 5 - Section 5.6 - Factoring Trinomials - Exercise Set - Page 303: 21

$(2x-3)^2$

The two numbers whose product is $ac= 4(9)=36 $ and whose sum is $b= -12 $ are $\{ -6,-6 \}$. Using these two numbers to decompose the middle term, then the factored form of the given expression, $ 4x^2-12x+9 $,is \begin{array}{l}\require{cancel} 4x^2-6x-6x+9 \\\\= (4x^2-6x)-(6x-9) \\\\= 2x(2x-3)-3(2x-3) \\\\= (2x-3)(2x-3) \\\\= (2x-3)^2 .\end{array}