Solve 8x − 9 = −81

The solution of 8x − 9 = −81 is x = -9.

8x - 9 = -81\ \Rightarrow\ x = -9

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The lines y = 8x − 9 and y = -81 meet at x = -9. 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.
$8x - 9 = -81$
2
Add 9 to both sides:
$8x - 9 + 9 = -81 + 9 \ \Rightarrow\ 8x = -72$
3
Divide both sides by 8:
$\frac{8x}{8} = \frac{-72}{8} \ \Rightarrow\ x = -9$
4
Check: put x = -9 back into the equation.
$8\cdot(-9) - 9 = -72 - 9 = -81 \checkmark$

LaTeX source of the formulas on this page

\[ 8x - 9 = -81\ \Rightarrow\ x = -9 \]
\[ 8x - 9 = -81 \]
\[ 8x - 9 + 9 = -81 + 9 \ \Rightarrow\ 8x = -72 \]
\[ \frac{8x}{8} = \frac{-72}{8} \ \Rightarrow\ x = -9 \]
\[ 8\cdot(-9) - 9 = -72 - 9 = -81 \checkmark \]

Trying values of x in 8x − 9 = −81

x	8x − 9	Equals -81?
-11	-97	no
-10	-89	no
-9	-81	yes
-8	-73	no
-7	-65	no

Questions

What is x in 8x − 9 = −81?

The solution of 8x − 9 = −81 is x = -9.

How do you solve 8x − 9 = −81?

Add 9 to both sides to get 8x = -72, then divide both sides by 8 to get x = -9.

How do I check the answer?

Replace x by -9: 8(-9) − 9 = -81, which is -81.

Solve 8x − 9 = −81

How to work it out

Trying values of x in 8x − 9 = −81

Questions

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