No of equivalence classes as per Myhill Nerode equivalence relation = No of States in Minimal DFA
So Draw the NFA and convert into Minimal DFA.
Strings reaching at each state in Minimal DFA, are distinguish from strings reaching at other states, that is what equivalence class mean here.
Other things about equivalence class is that, if x and y are two strings of same class( here reaching same state), and we append some strings z to then i.e, xz and yz, that will also reach to same equivalence class (either same or any other). Minimal DFA serve our purpose.
if we analyse the regular expression, we can see
first state, all rejecting strings, doesn't belong to L or given regular expression.
second state, all strings ending with b or bb, means ending with b, and accepts, (a+b)*b(∊+b)
Third state, all strings ending with ba, and accepts,(a+b)*ba.