How do you find the catenary sag?

Question

The part I am confused on is how to find the catenary sag. I have done my research and I'm still confused. Can someone please explain how to do this on part E?

Given:

Answer 1

If your equation is $y=a \cosh (\frac xa) +c$, the minimum is at $x=0$ and the poles are at $x=\pm 35$. The sag is then $y(35)-y(0)$-the vertical distance between the support and the low point in the middle.

Answer 2

25.8158 ended up being my answer for the sag.