1. **State the problem:** Solve the equation $$3x = 21$$. 2. **Formula and rules:** To solve for $x$, divide both sides of the equation by the coefficient of $x$, which is 3. 3. **Intermediate work:** $$3x = 21$$ Divide both sides by 3: $$\frac{\cancel{3}x}{\cancel{3}} = \frac{21}{3}$$ 4. **Simplify:** $$x = 7$$ 5. **Explanation:** We isolated $x$ by dividing both sides by 3, which cancels the 3 multiplying $x$. This leaves $x$ alone on one side, and the other side simplifies to 7. **Final answer:** $$x = 7$$