`(2y-1)^2 -(y+3)^2`
`= ( 4y^2 - 4y + 1) -(y^2 + 6y + 9)`
`= 4y^2 -4y +1 -y^2 -6y -9`
`= 3y^2 -10y -8`
\(\left(2y-1\right)^2-\left(y+3\right)^2\)
\(=2y\left(2y-1\right)-1\left(2y-1\right)-1\left(y^2+3y+3y+9\right)\)
\(=\left(y-4\right)\left(3y+2\right)\)