\(a,-64+\left(x-3\right)^3\)
\(=\left(-4\right)^3+\left(x-3\right)^3\)
\(=\left(-4+x-3\right)\left[\left(-4\right)^2-\left(-4\right)\left(x-3\right)+\left(x-3\right)^2\right]\)
\(=\left(x-7\right)\left(16+4x-12+x^2-6x+9\right)\)
\(=\left(x-7\right)\left(x^2-2x+13\right)\)