Find the Maclaurin series of the function $f(x) = (3 x^2) \sin (7 x)$.
$$f(x) = \displaystyle \sum_{n=0}^\infty c_n x^n )$$
Compute the following coefficients:
$c_3 =$
$c_4 =$
$c_5 =$
$c_6 =$
$c_7 =$