Shota tried to subtract two polynomials. His work is shown below:
$\begin{align}
&\phantom{=}(12a^2+3ab+4b^2)-(7a^2-5ab+b^2)
\\\\
&=12a^2+3ab+4b^2-7a^2+5ab-b^2&\text{Step }1
\\\\
&=(12+3)a^3+(3+4)b^3+(-7+5)a^3+(5-1)b^3&\text{Step }2
\\\\
&=15a^3+7b^3-2a^3+4b^3&\text{Step }3
\end{align}$
**At which step did Shota make an error, if at all?**
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