I think it's
(a+b)*(∈ + a) + (a+b)*(bb)
The first part will accept anything that ends with a or even null string which should be accepted as the minimum length string in L ={b} so for L complement it will be null.
The second part will ensure that whatever you generate should not end in odd number of b's . (Something that is not to be accepted by the complement of this language.