Your examples of type f:= x^6+ax+b
have galois group (use maple) S3 wr S3 of order 72.
    Solvable 6-ic polynomials are treated by Tom Hagedorn
 in J.Alg  (2000) 233 pp 704-757.
 Lesser degree polynomials are treated in the book by Dummit & Foote.
 John