Solve 9x − 15 = −78

The solution of 9x − 15 = −78 is x = -7.

9x - 15 = -78\ \Rightarrow\ x = -7

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The lines y = 9x − 15 and y = -78 meet at x = -7. 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.
$9x - 15 = -78$
2
Add 15 to both sides:
$9x - 15 + 15 = -78 + 15 \ \Rightarrow\ 9x = -63$
3
Divide both sides by 9:
$\frac{9x}{9} = \frac{-63}{9} \ \Rightarrow\ x = -7$
4
Check: put x = -7 back into the equation.
$9\cdot(-7) - 15 = -63 - 15 = -78 \checkmark$

LaTeX source of the formulas on this page

\[ 9x - 15 = -78\ \Rightarrow\ x = -7 \]
\[ 9x - 15 = -78 \]
\[ 9x - 15 + 15 = -78 + 15 \ \Rightarrow\ 9x = -63 \]
\[ \frac{9x}{9} = \frac{-63}{9} \ \Rightarrow\ x = -7 \]
\[ 9\cdot(-7) - 15 = -63 - 15 = -78 \checkmark \]

Trying values of x in 9x − 15 = −78

x	9x − 15	Equals -78?
-9	-96	no
-8	-87	no
-7	-78	yes
-6	-69	no
-5	-60	no

Questions

What is x in 9x − 15 = −78?

The solution of 9x − 15 = −78 is x = -7.

How do you solve 9x − 15 = −78?

Add 15 to both sides to get 9x = -63, then divide both sides by 9 to get x = -7.

How do I check the answer?

Replace x by -7: 9(-7) − 15 = -78, which is -78.

Solve 9x − 15 = −78

How to work it out

Trying values of x in 9x − 15 = −78

Questions

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