Solve 3x − 15 = 21

The solution of 3x − 15 = 21 is x = 12.

3x - 15 = 21\ \Rightarrow\ x = 12

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The lines y = 3x − 15 and y = 21 meet at x = 12. 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.
$3x - 15 = 21$
2
Add 15 to both sides:
$3x - 15 + 15 = 21 + 15 \ \Rightarrow\ 3x = 36$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{36}{3} \ \Rightarrow\ x = 12$
4
Check: put x = 12 back into the equation.
$3\cdot12 - 15 = 36 - 15 = 21 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 15 = 21\ \Rightarrow\ x = 12 \]
\[ 3x - 15 = 21 \]
\[ 3x - 15 + 15 = 21 + 15 \ \Rightarrow\ 3x = 36 \]
\[ \frac{3x}{3} = \frac{36}{3} \ \Rightarrow\ x = 12 \]
\[ 3\cdot12 - 15 = 36 - 15 = 21 \checkmark \]

Trying values of x in 3x − 15 = 21

x	3x − 15	Equals 21?
10	15	no
11	18	no
12	21	yes
13	24	no
14	27	no

Questions

What is x in 3x − 15 = 21?

The solution of 3x − 15 = 21 is x = 12.

How do you solve 3x − 15 = 21?

Add 15 to both sides to get 3x = 36, then divide both sides by 3 to get x = 12.

How do I check the answer?

Replace x by 12: 3(12) − 15 = 21, which is 21.

Solve 3x − 15 = 21

How to work it out

Trying values of x in 3x − 15 = 21

Questions

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