Represent the function $\displaystyle \frac{3 x}{7 + x}$ as a power series $f(x) = \displaystyle \sum_{n=0}^\infty c_n x^n$
$c_0 =$
$c_1 =$
$c_2 =$
$c_3 =$
$c_4 =$
Find the radius of convergence $R =$ .