1. Stating the problem: Solve the equation $x + 2x = 45$ for $x$. 2. Combine like terms on the left side: $x + 2x = 3x$. 3. The equation becomes $3x = 45$. 4. To isolate $x$, divide both sides by 3: $$x = \frac{45}{3}$$. 5. Simplify the right side: $$x = 15$$. 6. Therefore, the solution to the equation $x + 2x = 45$ is $x = 15$. Note: Although other equations were mentioned, only the first equation is solved as per instructions.