Solve 3x − 9 = 30

The solution of 3x − 9 = 30 is x = 13.

3x - 9 = 30\ \Rightarrow\ x = 13

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The lines y = 3x − 9 and y = 30 meet at x = 13. 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.
$3x - 9 = 30$
2
Add 9 to both sides:
$3x - 9 + 9 = 30 + 9 \ \Rightarrow\ 3x = 39$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{39}{3} \ \Rightarrow\ x = 13$
4
Check: put x = 13 back into the equation.
$3\cdot13 - 9 = 39 - 9 = 30 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 9 = 30\ \Rightarrow\ x = 13 \]
\[ 3x - 9 = 30 \]
\[ 3x - 9 + 9 = 30 + 9 \ \Rightarrow\ 3x = 39 \]
\[ \frac{3x}{3} = \frac{39}{3} \ \Rightarrow\ x = 13 \]
\[ 3\cdot13 - 9 = 39 - 9 = 30 \checkmark \]

Trying values of x in 3x − 9 = 30

x	3x − 9	Equals 30?
11	24	no
12	27	no
13	30	yes
14	33	no
15	36	no

Questions

What is x in 3x − 9 = 30?

The solution of 3x − 9 = 30 is x = 13.

How do you solve 3x − 9 = 30?

Add 9 to both sides to get 3x = 39, then divide both sides by 3 to get x = 13.

How do I check the answer?

Replace x by 13: 3(13) − 9 = 30, which is 30.

Solve 3x − 9 = 30

How to work it out

Trying values of x in 3x − 9 = 30

Questions

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