Equilibrium Distance Between Carts with Spring and Chain

[CornMuffin]

Homework Statement

Two carts can slide along a horizontal rail without friction. The carts are connected:
(a) by an elastic spring of spring constant k and unstretched length l;
(b) by a chain of length l and linear density p.
The spring is going along the rail, the chain hangs in the vertical plane.
[PLAIN]http://img823.imageshack.us/img823/3803/project1f.jpg
Find the equation for the equilibrium distance, d, between the carts.

Homework Equations

The Attempt at a Solution

In the y direction:
F=0
lpg=2TsinA where A is the angle the chain makes with the spring
T=lpg/(2sinA)

In the x direction:
F=0
k(l-d)=lpg/(2tanA)
d=l-lpg/(2ktanA)

If I can find an equation that models the chain, then I can find the angle, but I am having trouble with the equation.
A catenary can be used to model it, which I believe is in the form (but I can be wrong):
f(x)=acosh(x/a), but I don't know how to find a value for of a

 

[diazona]

You're right about the catenary,
$$f(x) = A \cosh\frac{x}{A}$$
Think about this: what do you know about the size of the catenary that might help you determine $A$?