G7.4 If $x+k$ is a factor of $3 x^{2}+14 x+a$, find $k$. ( $k$ is an integer.)
$\begin{array}{l}3(-k)^{2}+14(-k)+8=0 \\ \Rightarrow 3 k^{2}-14 k+8=0 \\ (3 k-2)(k-4)=0 \\ \left.\Rightarrow k=4 \quad \text { (reject } \frac{2}{3}\right)\end{array}$