A cable hangs between two poles of equal height and $20$ feet apart. Set up a coordinate system where the poles are placed at $x=-10$ and $x=10,$ where $x$ is measured in feet. The height $($in feet$)$ of the cable at position $x$ is

$h(x)=9cosh(\frac{x}{9})$

where $cosh(x)=\frac{e^x{e}^{-x}}{2}$ is the hyperbolic cosine, which is an important function in physics and engineering.

The cable is $$ feet long.