Solve 3x − 9 = 15

The solution of 3x − 9 = 15 is x = 8.

3x - 9 = 15\ \Rightarrow\ x = 8

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The lines y = 3x − 9 and y = 15 meet at x = 8. 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.
$3x - 9 = 15$
2
Add 9 to both sides:
$3x - 9 + 9 = 15 + 9 \ \Rightarrow\ 3x = 24$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{24}{3} \ \Rightarrow\ x = 8$
4
Check: put x = 8 back into the equation.
$3\cdot8 - 9 = 24 - 9 = 15 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 9 = 15\ \Rightarrow\ x = 8 \]
\[ 3x - 9 = 15 \]
\[ 3x - 9 + 9 = 15 + 9 \ \Rightarrow\ 3x = 24 \]
\[ \frac{3x}{3} = \frac{24}{3} \ \Rightarrow\ x = 8 \]
\[ 3\cdot8 - 9 = 24 - 9 = 15 \checkmark \]

Trying values of x in 3x − 9 = 15

x	3x − 9	Equals 15?
6	9	no
7	12	no
8	15	yes
9	18	no
10	21	no

Questions

What is x in 3x − 9 = 15?

The solution of 3x − 9 = 15 is x = 8.

How do you solve 3x − 9 = 15?

Add 9 to both sides to get 3x = 24, then divide both sides by 3 to get x = 8.

How do I check the answer?

Replace x by 8: 3(8) − 9 = 15, which is 15.

Solve 3x − 9 = 15

How to work it out

Trying values of x in 3x − 9 = 15

Questions

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