Are 6ab and 4ba like terms?

Yep! Because with a little rearrangement, this 4ba becomes 4ab. You can do this because, well, in multiplication this works. I mean, 2 x 3 is the same as 3 x 2 right? So to be like terms you have to have the same variables in both terms, and in both you have an a and a b being multiplied together- its just easier to see if you rearrange it (but not necessary)