Francesca tried to find the derivative of $-2x+9$ using basic differentiation rules. Here is her work:
$\begin{align}
&&&\phantom{=}\dfrac{d}{dx}(-2x+9)
\\\\
\text{Step 1}
&&&=\dfrac{d}{dx}(-2x)+\dfrac{d}{dx}(9)
\\\\
\text{Step 2}
&&&=\dfrac{d}{dx}(-2x)+9
\\\\
\text{Step 3}
&&&=-2\dfrac{d}{dx}(x)+9
\\\\
\text{Step 4}
&&&=-2\cdot 1+9
\\\\
\text{Step 5}
&&&=7
\end{align}$
**At which step did Francesca make a mistake, if at all?**
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