The activation energy for the reaction no2(g)+co(g)⟶no(g)+co2(g) is ea = 400 kj/mol and the change in enthalpy for the reaction is δh = -250 kj/mol . what is the activation energy for the reverse reaction?

If we are looking for the activation energy. You must know that it should be equal to the activation energy of the forward reaction deducted by the drive in the reaction which is delta E. But since it is the reverse, change the sign of delta E.
The computation for this would be: 400 kJ/mol + 250 kJ/mol = 650 kJ/mol is the activation energy for the reverse reaction.

They are all full and don't need any more electrons in there outer shell.