Find the value of the constant $c$ that makes the following function continuous on $(-\infty,\infty)$. $$f(y) = \begin{cases}\displaystyle{cy+2}&\text{if}\ y\in \left(-\infty ,6\right]\cr \displaystyle{cy^{2}-2}&\text{if}\ y\in \left(6,\infty \right)\end{cases}$$

$c=$