Calculate Lagoon Volume: x,y in km, D(x,y) = 40−160(y−f(x))2 − 40x2

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A river follows the path y = f(z) where x, y are in kilometers. Near the
sea, it widens into a lagoon, then narrows again at its mouth. At the point
(x, y), the depth of the lagoon is given by D(x, y) = 40−160(y−f(x))2 − 40x2 meters. The lagoon itself is described by D(x, y)  0. What is the
volume of the lagoon in cubic meters?

so far i got that this problem needs to use transformation and the jacobian.
i put x=u^2-v^2
y=2uv
find the trans using the matrix getting
4u^2+4v^2
double integral (2uv)(4u^2+4v^2)dudv
if what i did so far was right could anyone tell me how to find the limits or at least enlighten me in the right direction
 
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I'm sorry but I have no idea what "D(x, y)  0" means. What that "box" a special symbol?
 
sorry i wrote it down wrong
D(x,y)>=0
 
There are two things I don't understand about this problem. First, when finding the nth root of a number, there should in theory be n solutions. However, the formula produces n+1 roots. Here is how. The first root is simply ##\left(r\right)^{\left(\frac{1}{n}\right)}##. Then you multiply this first root by n additional expressions given by the formula, as you go through k=0,1,...n-1. So you end up with n+1 roots, which cannot be correct. Let me illustrate what I mean. For this...
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