Solve 3x − 9 = −21

The solution of 3x − 9 = −21 is x = -4.

3x - 9 = -21\ \Rightarrow\ x = -4

Explain it step by step in chat Make a short video of it

The lines y = 3x − 9 and y = -21 meet at x = -4. 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.
$3x - 9 = -21$
2
Add 9 to both sides:
$3x - 9 + 9 = -21 + 9 \ \Rightarrow\ 3x = -12$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{-12}{3} \ \Rightarrow\ x = -4$
4
Check: put x = -4 back into the equation.
$3\cdot(-4) - 9 = -12 - 9 = -21 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 9 = -21\ \Rightarrow\ x = -4 \]
\[ 3x - 9 = -21 \]
\[ 3x - 9 + 9 = -21 + 9 \ \Rightarrow\ 3x = -12 \]
\[ \frac{3x}{3} = \frac{-12}{3} \ \Rightarrow\ x = -4 \]
\[ 3\cdot(-4) - 9 = -12 - 9 = -21 \checkmark \]

Trying values of x in 3x − 9 = −21

x	3x − 9	Equals -21?
-6	-27	no
-5	-24	no
-4	-21	yes
-3	-18	no
-2	-15	no

Questions

What is x in 3x − 9 = −21?

The solution of 3x − 9 = −21 is x = -4.

How do you solve 3x − 9 = −21?

Add 9 to both sides to get 3x = -12, then divide both sides by 3 to get x = -4.

How do I check the answer?

Replace x by -4: 3(-4) − 9 = -21, which is -21.

Solve 3x − 9 = −21

How to work it out

Trying values of x in 3x − 9 = −21

Questions

Similar equations