--(equation 1)

Equation one should become --(equation 2)

Since you are given ----(equation 3)

For the 1st term of your equation 2, you plug in the derivative of the component of the equation 3, where .

For the 2st term of your equation 2, you plug in the derivative of the component of the equation 3,where .

The rest is algebraic work. For derivative with respect to x, hold y as constant, and for derivative with respect to y, hold x as constant