+26400 x + 9 = 18 + -2x
$$\begin{array}{rcl}
x + 9 &=& 18 + -2x \quad | \quad +2x \\
2x+x+9 &=& 18 \\
3x+9 &=& 18 \quad | \quad -9\\
3x &=& 18-9 \\
3x &=& 9 \quad | \quad :3\\
x &=& \frac{9}{3} \\
x &=& 3
\end{Array}
\text{Proof:} \\
3+9=18-2*3 \\
12 = 18-6 \\
12 = 12 \text{okay!}$$
