 Let’s call this relation R. It would take a lot of checking to actually verify that R is a bisimulation. Here is just two items of it:  We see that  2b R 2c. Now  2c → 2b.   Thus we need some node x so that x R 2b  and  2b → x. For this, we take  2a.     For our second point  of verification, again note that   2b R  2c .   Since   2b →  3b, we need some  node x so that  2c → x and  3b   R   x.  We take   x = 3c for this. 