Solve 3x − 9 = 42

The solution of 3x − 9 = 42 is x = 17.

3x - 9 = 42\ \Rightarrow\ x = 17

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The lines y = 3x − 9 and y = 42 meet at x = 17. 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.
$3x - 9 = 42$
2
Add 9 to both sides:
$3x - 9 + 9 = 42 + 9 \ \Rightarrow\ 3x = 51$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{51}{3} \ \Rightarrow\ x = 17$
4
Check: put x = 17 back into the equation.
$3\cdot17 - 9 = 51 - 9 = 42 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 9 = 42\ \Rightarrow\ x = 17 \]
\[ 3x - 9 = 42 \]
\[ 3x - 9 + 9 = 42 + 9 \ \Rightarrow\ 3x = 51 \]
\[ \frac{3x}{3} = \frac{51}{3} \ \Rightarrow\ x = 17 \]
\[ 3\cdot17 - 9 = 51 - 9 = 42 \checkmark \]

Trying values of x in 3x − 9 = 42

x	3x − 9	Equals 42?
15	36	no
16	39	no
17	42	yes
18	45	no
19	48	no

Questions

What is x in 3x − 9 = 42?

The solution of 3x − 9 = 42 is x = 17.

How do you solve 3x − 9 = 42?

Add 9 to both sides to get 3x = 51, then divide both sides by 3 to get x = 17.

How do I check the answer?

Replace x by 17: 3(17) − 9 = 42, which is 42.

Solve 3x − 9 = 42

How to work it out

Trying values of x in 3x − 9 = 42

Questions

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