Tangent modulus of the Mooney-Rivlin based Neo-hookean - Could you check my derivation??

Hello.

I study solid mechanics at a graduate school.

I tried to derive tangent modulus of the Mooney-Rivlin based Neo-Hookean material.

There might be no error in my derivation, but answer is wrong with referred right answer.

So, could you check my derivation?

I attached my derivation as a figure.