Answer
$(b-1)(b-4)$
Work Step by Step
The 2 numbers whose product is $ac=
1(4)=4
$ and whose sum is $b=
-5
$ are $\left\{
-1,-4
\right\}.$ Using these two numbers to decompose the middle term of the given trinomial, $
b^2-5b+4
,$ then the factored form is
\begin{array}{l}
b^2-1b-4b+4
\\\\=
(b^2-1b)-(4b-4)
\\\\=
b(b-1)-4(b-1)
\\\\=
(b-1)(b-4)
.\end{array}