JK flip flop analysis
"what is that A and B?"
The A's and B's usually refer to the output of the specific flip flop. In this analysis case, there are two JK flip flops with arbitrary names "A" and "B". The output's are also sometimes referred to as Q(A) and Q(B), where Q is the output and the respective subscript refers to the specific flip flop.
In the initial analysis question J=x and K=xB are the two excitation equations (the input equations) for the flip flop in question.
Q*=JQ' + K'Q is the characteristic equation for JK flip flops which describes how the flip flop arrives at its next state in algebraic terms. (Could be re-written as A*=JA' + K'A to clarify).
The initial post is missing the excitation equations for the second flip flop (assuming there are two since A and B appear in the equations). However, assuming the given excitation equations are for flip flop "A", we substitute J(A)=x and K(A)=xB into the characteristic equation which results in the next state equation for flip flop A:
A*= xA' + (XB)'A by DeMorgan's law, expand to get ...
A* = xA' + (x' + B')A expand ...
A* = xA' + x'A + B'A
It looks as if that answer: xA' + x'A + xB is wrong. I think that I am looking at this correctly, but without the schematic or more information, it's hard to know for sure if I'm missing something else.
-Justin