List these electron subshells in order of increasing energy. 6p , 7s , 6s , 4f note for advanced students: you may assume these subshells are all in an atom with many electrons, not a hydrogen atom.

             6s < 4f < 6p < 7s

Explanation:
                    The difference in energy of two subshells is decided by comparing their n+l value.
where;
           n  =  Principle Quantum Number (main Shell)

            l  =  Azimuthal Quantum Number

The greater the n+l value the greater is the energy of subshell. Also, if two subshells have same n+l value, the the subshell with lower n value will be having lesser energy.
So,
For,
                             6p  =  6 + 1  =  7

                             7s  =  7 + 0  =  7

                             6s  =  6 + 0  =  6

                             4f  =  4 + 3  =  7

Hence, 6s has the lowest energy, among 6p, 7s and 4f (all having same n+l values) 4f has lowest energy compared to 6p and 7s as it has lowest n value. And 6p has less energy than 7s.

Abbreviated as mg-25.