Solve 3x − 12 = 9

The solution of 3x − 12 = 9 is x = 7.

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

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The lines y = 3x − 12 and y = 9 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.
$3x - 12 = 9$
2
Add 12 to both sides:
$3x - 12 + 12 = 9 + 12 \ \Rightarrow\ 3x = 21$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{21}{3} \ \Rightarrow\ x = 7$
4
Check: put x = 7 back into the equation.
$3\cdot7 - 12 = 21 - 12 = 9 \checkmark$

LaTeX source of the formulas on this page

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

Trying values of x in 3x − 12 = 9

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

Questions

What is x in 3x − 12 = 9?

The solution of 3x − 12 = 9 is x = 7.

How do you solve 3x − 12 = 9?

Add 12 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) − 12 = 9, which is 9.

Solve 3x − 12 = 9

How to work it out

Trying values of x in 3x − 12 = 9

Questions

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