To solve the equation \( x + 2x = 21 \), we can follow these steps:

1. Combine like terms on the left-hand side:
   \[
   x + 2x = 3x
   \]
   So, the equation becomes:
   \[
   3x = 21
   \]

2. Now, solve for \( x \) by dividing both sides by 3:
   \[
   x = \frac{21}{3} = 7
   \]

Thus, \( x = 7 \).

Would you like more details on this, or have any other questions?

Here are 5 related questions to practice:
1. Solve \( 3x + 4x = 35 \).
2. Solve \( 5x + x = 18 \).
3. Solve \( x + 3x = 48 \).
4. Solve \( 4x + 6x = 100 \).
5. Solve \( 2x + 3x = 50 \).

**Tip:** Always combine like terms first when solving linear equations for a variable!

Learn how to solve the linear equation x + 2x = 21 step-by-step. This problem covers basic algebraic concepts like combining like terms and solving linear equations. Suitable for students in Grades 6-8.

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