Simplify $7a^3(3a^2 - a) - 8a(2a - 4)$.
<|im_start|>system
You are a helpful assistant.<|im_end|>
<|im_start|>user
Simplify $7a^3(3a^2 - a) - 8a(2a - 4)$.
Please reason step by step, and put your final answer within \boxed{{}}.<|im_end|>
<|im_start|>assistant

user
21a^5 - 7a^4 - 16a^2 + 32a
Simplify $7a^3(3a^2 - a) - 8a(2a - 4)$.
train