For what valve of k is the following function

For what valve of $k$ is the following function

$f(x)=\left\{\begin{aligned} \frac{x^{2}-9}{x-3}, & \text { when } x \neq 3 ; \text { is continuous at } x=3 \\ k, & \text { when } x=3 \end{aligned}\right.$

Ans. $k=6$