x + 9 = 18 + -2x

x + 9 = 18 + -2x

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