Solve 3x − 12 = 21

The solution of 3x − 12 = 21 is x = 11.

3x - 12 = 21\ \Rightarrow\ x = 11

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The lines y = 3x − 12 and y = 21 meet at x = 11. 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.
$3x - 12 = 21$
2
Add 12 to both sides:
$3x - 12 + 12 = 21 + 12 \ \Rightarrow\ 3x = 33$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{33}{3} \ \Rightarrow\ x = 11$
4
Check: put x = 11 back into the equation.
$3\cdot11 - 12 = 33 - 12 = 21 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 12 = 21\ \Rightarrow\ x = 11 \]
\[ 3x - 12 = 21 \]
\[ 3x - 12 + 12 = 21 + 12 \ \Rightarrow\ 3x = 33 \]
\[ \frac{3x}{3} = \frac{33}{3} \ \Rightarrow\ x = 11 \]
\[ 3\cdot11 - 12 = 33 - 12 = 21 \checkmark \]

Trying values of x in 3x − 12 = 21

x	3x − 12	Equals 21?
9	15	no
10	18	no
11	21	yes
12	24	no
13	27	no

Questions

What is x in 3x − 12 = 21?

The solution of 3x − 12 = 21 is x = 11.

How do you solve 3x − 12 = 21?

Add 12 to both sides to get 3x = 33, then divide both sides by 3 to get x = 11.

How do I check the answer?

Replace x by 11: 3(11) − 12 = 21, which is 21.

Solve 3x − 12 = 21

How to work it out

Trying values of x in 3x − 12 = 21

Questions

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