You insert kronecker deltas: ∂M_ab / ∂M_cd = δ_ac δ_bd , and a,b here remain contracted with whatever M was originally contracted with. Then you simplify, δ_ac

You insert kronecker deltas: ∂M_ab / ∂M_cd = δ_ac δ_bd , and a,b here remain contracted with whatever M was originally contracted with. Then you simplify, δ_ac Z_xya = Z_xyc and so on.