`xx^2`+9x+20

=`xx^2`+5x +4x +20

=x(x+5)+(x+4)

=(x+5)  (x+4)

`xx^2` -2x -15
=`xx^2` -5x + 3x -15
=x(x-5)+3(x-5)
=(x-5) (x+3)

6`xx^2` + 11xy - 10`y^2`
=6`xx^2`+ 15xy - 4xy - 10`y^2`
=3x(2x +5y) -2(2x+5y)
=(3x-2y) (2x+5y)

`y^2`- y - 42 
=`y^2`- 7y + 6y - 42
=y(y+7) + 6(y-7)
=(y-7) (y+6)

`x^2` - `y^2`
=(x+y) (x-y)
`:.` `100^2` - `99^2`
=(100+99) (100-99)
=199+1
=200