Question 1198978
<pre>
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.

{{{matrix(8,1,

a(3a+1)-(2a+1)(a-1),
3a^2+a-(2a^2-2a+a-1),
3a^2+a-(2a^2-a-1),
3a^2+a-2a^2+a+1,
3a^2-2a^2+a+a+1,
a^2+2a+1,
(a+1)(a+1),
(a+1)^2)}}}

Edwin</pre>