Answer
$n=9$
Work Step by Step
Given $a_{n}=4n^{2}+3n$ and $a_{n}=351$
$\implies 4n^{2}+3n=351$
$\implies 4n^{2}+3n-351=0$
The above equation is a quadratic equation and can be solved for $n$.
$n=\frac{-3+ \sqrt {3^{2}-4(4)(-351)}}{2(4)}=9$
(Negative-value solution of the quadratic equation is neglected because $n$ cannot be negative.)
