A cable hangs between two poles of equal height and 30$30$ feet apart. Set up a coordinate system where the poles are placed at x=−15 and x=15, where x is measured in feet. The height (in feet) of the cable at position x is

h(x)=15cosh(x/15)$,$

where cosh(x)=(e^x+e^−x)/2 is the hyperbolic cosine, which is an important function in physics and engineering.

The cable is __ feet long.