Factorise:

$21 x^{2}-2 x+\frac{1}{21}=21 x^{2}-x-x+\frac{1}{21}$

$=21 x\left(x-\frac{1}{21}\right)-1\left(x-\frac{1}{21}\right)$

$=\left(x-\frac{1}{21}\right)(21 x-1)$

Hence, factorisation of $21 x^{2}-2 x+\frac{1}{21}$ is $\left(x-\frac{1}{21}\right)(21 x-1)$