How do you factor #x(b-5)+z(b-5) #?
(b - 5)(x +z)
There is a common factor of (b -5) in the 2 terms which can be taken out.
When (b - 5) is taken out of the 1st term we are left with x.
Similarly, when (b -5) is taken out of the 2nd term we are left with z.
hence (b -5)(x + z) are the factors.