Find the values of $x$ (if any) at which $f$ is not continuous, and determine whether each such value is a removable discontinuity.
$$
\begin{array}{ll}{\text { (a) } f(x)=\frac{x^{2}-4}{x^{3}-8}} & {\text { (b) } f(x)=\left\{\begin{array}{ll}{2 x-3,} & {x \leq 2} \\ {x^{2},} & {x>2}\end{array}\right.} \\ {\text { (c) } f(x)=\left\{\begin{array}{ll}{3 x^{2}+5,} & {x \neq 1} \\ {6,} & {x=1}\end{array}\right.}\end{array}
$$