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Solve 4x + 9 = 81

The solution of 4x + 9 = 81 is x = 18.

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The lines y = 4x + 9 and y = 81 meet at x = 181214161820222460708090100110x = 18y = 4x + 9y = 81
The lines y = 4x + 9 and y = 81 meet at x = 18. 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.

    4x+9=814x + 9 = 81
  2. 2

    Subtract 9 from both sides:

    4x+9−9=81−9 ⇒ 4x=724x + 9 - 9 = 81 - 9 \ \Rightarrow\ 4x = 72
  3. 3

    Divide both sides by 4:

    4x4=724 ⇒ x=18\frac{4x}{4} = \frac{72}{4} \ \Rightarrow\ x = 18
  4. 4

    Check: put x = 18 back into the equation.

    4⋅18+9=72+9=81✓4\cdot18 + 9 = 72 + 9 = 81 \checkmark
LaTeX source of the formulas on this page
\[ 4x + 9 = 81\ \Rightarrow\ x = 18 \]
\[ 4x + 9 = 81 \]
\[ 4x + 9 - 9 = 81 - 9 \ \Rightarrow\ 4x = 72 \]
\[ \frac{4x}{4} = \frac{72}{4} \ \Rightarrow\ x = 18 \]
\[ 4\cdot18 + 9 = 72 + 9 = 81 \checkmark \]

Trying values of x in 4x + 9 = 81

x4x + 9Equals 81?
1673no
1777no
1881yes
1985no
2089no

Questions

What is x in 4x + 9 = 81?

The solution of 4x + 9 = 81 is x = 18.

How do you solve 4x + 9 = 81?

Subtract 9 from both sides to get 4x = 72, then divide both sides by 4 to get x = 18.

How do I check the answer?

Replace x by 18: 4(18) + 9 = 81, which is 81.

Similar equations

    Solve 4x + 9 = 81: x = 18 (steps and check) | WordGPT