I find that perhaps c=160/2 which would solve for y...

I also suppose that a line from the centre to the corner would be bisected by Z and therefore provide another known relationship within the kite that has Z as the central axis...

Here is another scan of a scale (but not perfect) diagram of the problem showing as much as I can what is known and what is supposed>?

Note that the image has been slightly cropped at the bottom and left - the total page size is 320mm x 450mm and therefore the centre is known to be at 160x225. The top right corner is assumed to be a right angle and it is folded down to the centre point to create the line Z which is the key length to find.