An equation can be rewritten by changing all signs. This is equivalent to multiply or divide both sides by $\text{-}1.$ $\begin{aligned} \text{-} x=3 \quad &\Leftrightarrow \quad x=\text{-} 3 \\ \text{-} x^2 -7x+4=0 \quad &\Leftrightarrow \quad x^2+7x-4=0 \end{aligned}$