Solve 3x − 9 = 12

The solution of 3x − 9 = 12 is x = 7.

3x - 9 = 12\ \Rightarrow\ x = 7

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The lines y = 3x − 9 and y = 12 meet at x = 7. The solution of the equation is the x-coordinate of the point where the two sides are equal.

How to work it out

1
The goal is to get x alone. Undo the operations in the opposite order: first the subtraction, then the multiplication.
$3x - 9 = 12$
2
Add 9 to both sides:
$3x - 9 + 9 = 12 + 9 \ \Rightarrow\ 3x = 21$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{21}{3} \ \Rightarrow\ x = 7$
4
Check: put x = 7 back into the equation.
$3\cdot7 - 9 = 21 - 9 = 12 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 9 = 12\ \Rightarrow\ x = 7 \]
\[ 3x - 9 = 12 \]
\[ 3x - 9 + 9 = 12 + 9 \ \Rightarrow\ 3x = 21 \]
\[ \frac{3x}{3} = \frac{21}{3} \ \Rightarrow\ x = 7 \]
\[ 3\cdot7 - 9 = 21 - 9 = 12 \checkmark \]

Trying values of x in 3x − 9 = 12

x	3x − 9	Equals 12?
5	6	no
6	9	no
7	12	yes
8	15	no
9	18	no

Questions

What is x in 3x − 9 = 12?

The solution of 3x − 9 = 12 is x = 7.

How do you solve 3x − 9 = 12?

Add 9 to both sides to get 3x = 21, then divide both sides by 3 to get x = 7.

How do I check the answer?

Replace x by 7: 3(7) − 9 = 12, which is 12.

Solve 3x − 9 = 12

How to work it out

Trying values of x in 3x − 9 = 12

Questions

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