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Solve 3x + 9 = 21

The solution of 3x + 9 = 21 is x = 4.

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The lines y = 3x + 9 and y = 21 meet at x = 4-2246810510152025303540x = 4y = 3x + 9y = 21
The lines y = 3x + 9 and y = 21 meet at x = 4. The solution of the equation is the x-coordinate of the point where the two sides are equal.

How to work it out

  1. 1

    The goal is to get x alone. Undo the operations in the opposite order: first the addition, then the multiplication.

    3x+9=213x + 9 = 21
  2. 2

    Subtract 9 from both sides:

    3x+9−9=21−9 ⇒ 3x=123x + 9 - 9 = 21 - 9 \ \Rightarrow\ 3x = 12
  3. 3

    Divide both sides by 3:

    3x3=123 ⇒ x=4\frac{3x}{3} = \frac{12}{3} \ \Rightarrow\ x = 4
  4. 4

    Check: put x = 4 back into the equation.

    3⋅4+9=12+9=21✓3\cdot4 + 9 = 12 + 9 = 21 \checkmark
LaTeX source of the formulas on this page
\[ 3x + 9 = 21\ \Rightarrow\ x = 4 \]
\[ 3x + 9 = 21 \]
\[ 3x + 9 - 9 = 21 - 9 \ \Rightarrow\ 3x = 12 \]
\[ \frac{3x}{3} = \frac{12}{3} \ \Rightarrow\ x = 4 \]
\[ 3\cdot4 + 9 = 12 + 9 = 21 \checkmark \]

Trying values of x in 3x + 9 = 21

x3x + 9Equals 21?
215no
318no
421yes
524no
627no

Questions

What is x in 3x + 9 = 21?

The solution of 3x + 9 = 21 is x = 4.

How do you solve 3x + 9 = 21?

Subtract 9 from both sides to get 3x = 12, then divide both sides by 3 to get x = 4.

How do I check the answer?

Replace x by 4: 3(4) + 9 = 21, which is 21.

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    Solve 3x + 9 = 21: x = 4 (steps and check) | WordGPT