Solve 9x − 7 = 128

The solution of 9x − 7 = 128 is x = 15.

9x - 7 = 128\ \Rightarrow\ x = 15

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The lines y = 9x − 7 and y = 128 meet at x = 15. 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.
$9x - 7 = 128$
2
Add 7 to both sides:
$9x - 7 + 7 = 128 + 7 \ \Rightarrow\ 9x = 135$
3
Divide both sides by 9:
$\frac{9x}{9} = \frac{135}{9} \ \Rightarrow\ x = 15$
4
Check: put x = 15 back into the equation.
$9\cdot15 - 7 = 135 - 7 = 128 \checkmark$

LaTeX source of the formulas on this page

\[ 9x - 7 = 128\ \Rightarrow\ x = 15 \]
\[ 9x - 7 = 128 \]
\[ 9x - 7 + 7 = 128 + 7 \ \Rightarrow\ 9x = 135 \]
\[ \frac{9x}{9} = \frac{135}{9} \ \Rightarrow\ x = 15 \]
\[ 9\cdot15 - 7 = 135 - 7 = 128 \checkmark \]

Trying values of x in 9x − 7 = 128

x	9x − 7	Equals 128?
13	110	no
14	119	no
15	128	yes
16	137	no
17	146	no

Questions

What is x in 9x − 7 = 128?

The solution of 9x − 7 = 128 is x = 15.

How do you solve 9x − 7 = 128?

Add 7 to both sides to get 9x = 135, then divide both sides by 9 to get x = 15.

How do I check the answer?

Replace x by 15: 9(15) − 7 = 128, which is 128.

Solve 9x − 7 = 128

How to work it out

Trying values of x in 9x − 7 = 128

Questions

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