Solve 3x + 9 = −21

The solution of 3x + 9 = −21 is x = -10.

3x + 9 = -21\ \Rightarrow\ x = -10

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The lines y = 3x + 9 and y = -21 meet at x = -10. 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 addition, then the multiplication.
$3x + 9 = -21$
2
Subtract 9 from both sides:
$3x + 9 - 9 = -21 - 9 \ \Rightarrow\ 3x = -30$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{-30}{3} \ \Rightarrow\ x = -10$
4
Check: put x = -10 back into the equation.
$3\cdot(-10) + 9 = -30 + 9 = -21 \checkmark$

LaTeX source of the formulas on this page

\[ 3x + 9 = -21\ \Rightarrow\ x = -10 \]
\[ 3x + 9 = -21 \]
\[ 3x + 9 - 9 = -21 - 9 \ \Rightarrow\ 3x = -30 \]
\[ \frac{3x}{3} = \frac{-30}{3} \ \Rightarrow\ x = -10 \]
\[ 3\cdot(-10) + 9 = -30 + 9 = -21 \checkmark \]

Trying values of x in 3x + 9 = −21

x	3x + 9	Equals -21?
-12	-27	no
-11	-24	no
-10	-21	yes
-9	-18	no
-8	-15	no

Questions

What is x in 3x + 9 = −21?

The solution of 3x + 9 = −21 is x = -10.

How do you solve 3x + 9 = −21?

Subtract 9 from both sides to get 3x = -30, then divide both sides by 3 to get x = -10.

How do I check the answer?

Replace x by -10: 3(-10) + 9 = -21, which is -21.

Solve 3x + 9 = −21

How to work it out

Trying values of x in 3x + 9 = −21

Questions

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