i am stucked with a comment 'The variable is undefined . check that the label is set correctly.'

We don't see if you defined b and if you did, we don't see how you did it.

Its hard to impossible to debug a picture.

If possible please post the worksheet.

Hi Saru.

Although the Viktor answer seems to me very correct, the error message in your function is just because b it's wrong: it could be a[1 instead.

Best regards.

Alvaro.

Solving with partial derivatived expressions.

Viktor