a: =5x^4+x=x(5x^3+1)
b: =(x-3)^2-y^2
=(x-3-y)(x-3+y)
\(4x^4+4x^4+x\)
\(=8x^4+x\)
\(=x\left(8x^3+1\right)\)
\(=x\left[\left(2x\right)^3+1^3\right]\)
\(=x\left(2x+1\right)\left(4x^2-2x+1\right)\)
------------------------------------
\(x^2-6x-y^2+9\)
\(=\left(x^2-6x+9\right)-y^2\)
\(=\left(x-3\right)^2-y^2\)
\(=\left(x-y-3\right)\left(x+y-3\right)\)