Chapter 5 - Sections 5.1-5.7 - Integrated Review - Operations on Polynomials and Factoring Strategies - Page 313: 44

$(3y-7)^2$

The two numbers whose product is $441$ and whose sum is $-42$ are $\{-21,-21\}$. Using these numbers to decompose the middle term of the trinomial results to \begin{array}{l} 9y^2-42y+49 \\= 9y^2-21y-21y+49 \\= (9y^2-21y)-(21y-49) \\= 3y(3y-7)-7(3y-7) \\= (3y-7)(3y-7) \\= (3y-7)^2 .\end{array}