Isatou tried to subtract two polynomials. Her work is shown below:
$\begin{align}
&\phantom{=}(3c^2-5cd+d^2)-(7c^2+4cd-9d^2)
\\\\
&=3c^2-5cd+d^2-7c^2+4cd-9d^2&\text{Step }1
\\\\
&=(3-7)c^2+(-5+4)cd+(1-9)d^2&\text{Step }2
\\\\
&=-4c^2-cd-8d^2&\text{Step }3
\end{align}$
**At which step did Isatou make an error, if at all?**
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