Let's solve the equation step by step:

Given:
$3x + 9 = x + 21$

Step 1: Move all terms involving $x$ to one side.

Subtract $x$ from both sides: $3x - x + 9 = 21$ This simplifies to: $2x + 9 = 21$

Step 2: Isolate the term with $x$.

Subtract 9 from both sides: $2x = 21 - 9$ $2x = 12$

Step 3: Solve for $x$.

Divide both sides by 2: $x = \frac{12}{2} = 6$

So, the solution is $x = 6$.

Would you like further details on any part of the process or have any questions?

Here are five related questions:

1. How do you solve equations with fractions involved?
2. What happens if the variable appears on both sides of the equation with coefficients?
3. Can you solve an inequality instead of an equation?
4. How do you check if the solution of an equation is correct?
5. What are some methods to solve quadratic equations?

Tip: Always check your solution by substituting the value of $x$ back into the original equation to verify it's correct.