Sanaz tried to subtract two polynomials. Her work is shown below:
$\begin{align}
&\phantom{=}(-3a^4-7a^2b^2+6b^4)-(2a^4-3a^2b^2+4b^4)
\\\\
&=-3a^4-7a^2b^2+6b^4-2a^4+3a^2b^2-4b^4&\text{Step }1
\\\\
&=(-3-7)a^6b^2+(6-2)a^4b^4+(3-4)a^2b^6&\text{Step }2
\\\\
&=-10a^6b^2+4a^4b^4-a^2b^6&\text{Step }3
\end{align}$
**At which step did Sanaz make an error, if at all?**
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