Solve 7x − 15 = −78

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

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

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

LaTeX source of the formulas on this page

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

Trying values of x in 7x − 15 = −78

x	7x − 15	Equals -78?
-11	-92	no
-10	-85	no
-9	-78	yes
-8	-71	no
-7	-64	no

Questions

What is x in 7x − 15 = −78?

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

How do you solve 7x − 15 = −78?

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

How do I check the answer?

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

Solve 7x − 15 = −78

How to work it out

Trying values of x in 7x − 15 = −78

Questions

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