Hello!
The prime factors of these two numbres are :
[tex]25b^2 = 5^2\times b^2\\
20b = 5\times 2^2 \times b[/tex]
The GCF can be calculated by multiplying common primes factors :
[tex]5\times b = 5b[/tex]
So the GCF is 5b and the expression can be factored this way :
[tex]25b^2 - 20b = 5b\times 5b - 4\times 5b = 5b\left(5b-4\right)[/tex]
