Why does the second line follow from the first in covariant Maxwell equations?

[jennyjones]

can some tell me why the second line follows from the first in the attachment

thank you

 

[Orodruin]

It doesn't. The two sides of the equation have different free indices.

 

[jennyjones]

It doesn't. The two sides of the equation have different free indices.

thanx!

 

[Matterwave]

For some reason they have mad a swap $\mu\leftrightarrow\nu$ in the second line...and also switch the order of the partial derivatives.