As the coefficient of x2 is 1, you just have to look for factors of 20 which differ by 1.
These are obviously 4 and 5 so: (x - 5)(x + 4)
X squared + 7X + 12?
It is: x(x-3) factorized
x(x - 7)
(x - 6)(x + 1)
x(x-7)
7x-x2 = x(7-x)
What about it? Factorise it?
x2+7x x(x+7)
x2 + 4x = x(x + 4)
13X squared + 7X + 12
X2 - 4= (x + 2)(x - 2)===========
(x + 2)(x + 1)