f(x)=x^3-x^2-26x+23, k=5
Find the seros of the cubic equation
using a graphing tool
see the attached figure
please wait a minute
The zeros are -5, 1 and 5
so
f(x)=(x+5)(x+1)(x-5)
Find the product of (x+5)(x+1)
(x+5)(x+1)=x^2+x+5x+5
=(x^2+6x+5)
substitute
f(x)=(x-5)(x^2+6x+5)-7
