The polynomial 
may have solutions which are the divisors of -20, therefore -20 has the following divisors: 
.
If x=1, then
,
if x=-1, then
,
if x=2, then
, then x=2 is a solution and you have the first factor (x-2).
If x=-2, then
, then x=-2 is a solution, so you have the second factor (x+2).
Since x-2 and x+2 are two factors of , then the polynomial 
is a divisor of and dividing the polynomial by you obtain
.
If x=1, then
if x=-1, then
if x=2, then
If x=-2, then
Since x-2 and x+2 are two factors of