A rabbit has four methods, but because two rabbits can't go in and out from the same mouth, B rabbit has only three methods. Using the principle of multiplication, two rabbits have the method of 4x3 = 12.
Also, because two rabbits come in and out of this cave from different mouths, it is impossible for a rabbit to go out of the original cave. There are three ways out. Rabbit B can neither export from its own import nor export from Rabbit A, so there are only two ways to enter, and two rabbits have 3x2 = 6 ways to exit.
12 (incoming method) x6 (outgoing method) = 84 methods.
I suggest you take a look at the questions about the principle of multiplication and addition principle.