Solve 5x − 9 = 81

The solution of 5x − 9 = 81 is x = 18.

5x - 9 = 81\ \Rightarrow\ x = 18

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The lines y = 5x − 9 and y = 81 meet at x = 18. 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.
$5x - 9 = 81$
2
Add 9 to both sides:
$5x - 9 + 9 = 81 + 9 \ \Rightarrow\ 5x = 90$
3
Divide both sides by 5:
$\frac{5x}{5} = \frac{90}{5} \ \Rightarrow\ x = 18$
4
Check: put x = 18 back into the equation.
$5\cdot18 - 9 = 90 - 9 = 81 \checkmark$

LaTeX source of the formulas on this page

\[ 5x - 9 = 81\ \Rightarrow\ x = 18 \]
\[ 5x - 9 = 81 \]
\[ 5x - 9 + 9 = 81 + 9 \ \Rightarrow\ 5x = 90 \]
\[ \frac{5x}{5} = \frac{90}{5} \ \Rightarrow\ x = 18 \]
\[ 5\cdot18 - 9 = 90 - 9 = 81 \checkmark \]

Trying values of x in 5x − 9 = 81

x	5x − 9	Equals 81?
16	71	no
17	76	no
18	81	yes
19	86	no
20	91	no

Questions

What is x in 5x − 9 = 81?

The solution of 5x − 9 = 81 is x = 18.

How do you solve 5x − 9 = 81?

Add 9 to both sides to get 5x = 90, then divide both sides by 5 to get x = 18.

How do I check the answer?

Replace x by 18: 5(18) − 9 = 81, which is 81.

Solve 5x − 9 = 81

How to work it out

Trying values of x in 5x − 9 = 81

Questions

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