I've been told that Maxwell’s equations in the curved space-time $(\mathscr{M},g)$ take the form
$$\nabla^a F_{ab} =0 \, \,(*), \quad \nabla_a F_{bc} + \nabla_b F_{ca} + \nabla_c F_{ab} = 0 \, \,(**)$$
where $F_{ab} = -F_{ba}$ are components of a skew tensor of type $(0,2)$. I need to prove
$$\nabla^a \nabla_a F_{bc} -R_{bcas}F^{as} -r^s_bF_{cs} + r_c^s F_{bs} = 0 $$
My first guess is to operate on $(**)$ with $\nabla^a$, but after that I have no clue how to approach this.
Any help would be appreciated.