1. **State the problem:** Solve for the constant $x$ in the equation $$18 = 3x + 12$$. 2. **Formula and rules:** To isolate $x$, we use basic algebraic operations: subtraction and division. 3. **Step 1:** Subtract 12 from both sides to get rid of the constant term on the right: $$18 - 12 = 3x + 12 - 12$$ which simplifies to $$6 = 3x$$. 4. **Step 2:** Divide both sides by 3 to solve for $x$: $$\frac{6}{3} = \frac{3x}{3}$$ which simplifies to $$2 = x$$. 5. **Final answer:** The constant $x$ is $2$.