[SOLVED] Working out the volume of a cuboid

Here is the information I've been given:

The sum of the lengths of the 12 edges of a cuboid is cm. The distance from one corner of the cuboid to the furthest corner is cm. What is the total surface area of the cube in terms of and .

This is what I've got so far:

Call the three lengths on the cuboid .

The total surface area =

As is the longest diagonal,

And finally, as is the total lengths,

I've tried to re-arrange those equations to get the surface area in terms of and , but to no avail.

All help appreciated,

David

EDIT: I've just realised that I've put this in the wrong sub-forum. Please could one of the moderators move this to the appropriate sub-forum?