Simplify g^2-20gh-g+20h.

The first step is to create groups of
terms:

g^2-20gh-g+20h =
(g^2-20gh)-(g-20h)

We'll factorize by g the first
group:

g^2-20gh-g+20h = g(g - 20h) - (g -
20h)

We'll factorize by (g -
20h):

g^2-20gh-g+20h =  (g - 20h)(g -
1)

The simplified expresison is:
g^2-20gh-g+20h = (g - 20h)(g - 1)