Answer
$3d(2d-7)$.
Work Step by Step
The given expression is
$=6d^2-21d$
The GCF of $6$ and $21$ is $3$.
The GCF of $d^2$ and $d$ is $d$.
So, the greatest common monomial factor of the terms is $3d$.
Hence, the factor form is $3d(2d-7)$.