a) \(=\left(x-3\right)\left(x+3\right)\)
b) \(=\left(x^3-y^3\right)\left(x^3+y^3\right)=\left(x-y\right)\left(x+y\right)\left(x^2-xy+y^2\right)\left(x^2+xy+y^2\right)\)
c) \(=\left(3x+y\right)^2\)
d) \(=\left(x+2y\right)^2\)
f) \(=\left(3x+1-x-1\right)\left(3x+1+x+1\right)=4x\left(2x+1\right)\)
e) \(=\left(x+y-x+y\right)\left(x+y+x-y\right)=4xy\)

