Find the four second partial derivatives. Observe that the second mixed partials are equal.

$\frac{de{l}^{2}z}{del{x}^2}=$

$\frac{de{l}^{2}z}{\text{d}\text{e}\text{l}\text{y}\text{d}\text{e}\text{l}\text{x}}=$

$\frac{de{l}^{2}z}{del{y}^2}=$

$\frac{de{l}^{2}z}{\text{d}\text{e}\text{l}\text{x}\text{d}\text{e}\text{l}\text{y}}=$