Use the formula
y + z = 34XlengthXgirth divided by x - z.
This is usually how I calculate the weight of my fish....I haven't caught a fish under 50 pounds yet.
...there is a formula for determing the weight by certain measurements, I for one...don't buy it. It may be close...but there are other variables to consider. Buy a scale is my best guess....sorry, no pun intended.