When you got to -(2a+1)(a-1), since it is preceded by a "-", 
you should have kept the "FOIL"ing process inside parentheses, 
because it's easy to make a mistake to FOIL and change signs
all in one step, as you did. You forgot to change the sign of -1.
As I used to tell my students, "It's better
to get writer's cramp than to miss the problem". lol 

You also made an error combining the terms +1a+2a-1a which 
should have been 2a, not 4a.



Edwin