Find the Maclaurin series of the function $f(x) = (3 x) \arctan (7 x^2)$.
$$f(x) = \sum_{n=0}^\infty c_n x^n$$ Determine the following coefficients:
$c_3 =$
$c_5 =$
$c_7 =$
$c_9 =$
$c_{11} =$