Solve 3x − 12 = −21

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

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

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The lines y = 3x − 12 and y = -21 meet at x = -3. 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:
$3x - 12 + 12 = -21 + 12 \ \Rightarrow\ 3x = -9$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{-9}{3} \ \Rightarrow\ x = -3$
4
Check: put x = -3 back into the equation.
$3\cdot(-3) - 12 = -9 - 12 = -21 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 12 = -21\ \Rightarrow\ x = -3 \]
\[ 3x - 12 = -21 \]
\[ 3x - 12 + 12 = -21 + 12 \ \Rightarrow\ 3x = -9 \]
\[ \frac{3x}{3} = \frac{-9}{3} \ \Rightarrow\ x = -3 \]
\[ 3\cdot(-3) - 12 = -9 - 12 = -21 \checkmark \]

Trying values of x in 3x − 12 = −21

x	3x − 12	Equals -21?
-5	-27	no
-4	-24	no
-3	-21	yes
-2	-18	no
-1	-15	no

Questions

What is x in 3x − 12 = −21?

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

How do you solve 3x − 12 = −21?

Add 12 to both sides to get 3x = -9, then divide both sides by 3 to get x = -3.

How do I check the answer?

Replace x by -3: 3(-3) − 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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