Simplify $(a-1)(a+1)(a+2) - (a-2)(a+1).$
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Simplify $(a-1)(a+1)(a+2) - (a-2)(a+1).$
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
a^3 + a^2
Simplify $(a-1)(a+1)(a+2) - (a-2)(a+1).$
train