Solve 3x − 15 = 39

The solution of 3x − 15 = 39 is x = 18.

3x - 15 = 39\ \Rightarrow\ x = 18

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The lines y = 3x − 15 and y = 39 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.
$3x - 15 = 39$
2
Add 15 to both sides:
$3x - 15 + 15 = 39 + 15 \ \Rightarrow\ 3x = 54$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{54}{3} \ \Rightarrow\ x = 18$
4
Check: put x = 18 back into the equation.
$3\cdot18 - 15 = 54 - 15 = 39 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 15 = 39\ \Rightarrow\ x = 18 \]
\[ 3x - 15 = 39 \]
\[ 3x - 15 + 15 = 39 + 15 \ \Rightarrow\ 3x = 54 \]
\[ \frac{3x}{3} = \frac{54}{3} \ \Rightarrow\ x = 18 \]
\[ 3\cdot18 - 15 = 54 - 15 = 39 \checkmark \]

Trying values of x in 3x − 15 = 39

x	3x − 15	Equals 39?
16	33	no
17	36	no
18	39	yes
19	42	no
20	45	no

Questions

What is x in 3x − 15 = 39?

The solution of 3x − 15 = 39 is x = 18.

How do you solve 3x − 15 = 39?

Add 15 to both sides to get 3x = 54, then divide both sides by 3 to get x = 18.

How do I check the answer?

Replace x by 18: 3(18) − 15 = 39, which is 39.

Solve 3x − 15 = 39

How to work it out

Trying values of x in 3x − 15 = 39

Questions

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