Solve 3x − 9 = 51

The solution of 3x − 9 = 51 is x = 20.

3x - 9 = 51\ \Rightarrow\ x = 20

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The lines y = 3x − 9 and y = 51 meet at x = 20. 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.
$3x - 9 = 51$
2
Add 9 to both sides:
$3x - 9 + 9 = 51 + 9 \ \Rightarrow\ 3x = 60$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{60}{3} \ \Rightarrow\ x = 20$
4
Check: put x = 20 back into the equation.
$3\cdot20 - 9 = 60 - 9 = 51 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 9 = 51\ \Rightarrow\ x = 20 \]
\[ 3x - 9 = 51 \]
\[ 3x - 9 + 9 = 51 + 9 \ \Rightarrow\ 3x = 60 \]
\[ \frac{3x}{3} = \frac{60}{3} \ \Rightarrow\ x = 20 \]
\[ 3\cdot20 - 9 = 60 - 9 = 51 \checkmark \]

Trying values of x in 3x − 9 = 51

x	3x − 9	Equals 51?
18	45	no
19	48	no
20	51	yes
21	54	no
22	57	no

Questions

What is x in 3x − 9 = 51?

The solution of 3x − 9 = 51 is x = 20.

How do you solve 3x − 9 = 51?

Add 9 to both sides to get 3x = 60, then divide both sides by 3 to get x = 20.

How do I check the answer?

Replace x by 20: 3(20) − 9 = 51, which is 51.

Solve 3x − 9 = 51

How to work it out

Trying values of x in 3x − 9 = 51

Questions

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