Solve 3x − 15 = −42

The solution of 3x − 15 = −42 is x = -9.

3x - 15 = -42\ \Rightarrow\ x = -9

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The lines y = 3x − 15 and y = -42 meet at x = -9. 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.
$3x - 15 = -42$
2
Add 15 to both sides:
$3x - 15 + 15 = -42 + 15 \ \Rightarrow\ 3x = -27$
3
Divide both sides by 3:
$\frac{3x}{3} = \frac{-27}{3} \ \Rightarrow\ x = -9$
4
Check: put x = -9 back into the equation.
$3\cdot(-9) - 15 = -27 - 15 = -42 \checkmark$

LaTeX source of the formulas on this page

\[ 3x - 15 = -42\ \Rightarrow\ x = -9 \]
\[ 3x - 15 = -42 \]
\[ 3x - 15 + 15 = -42 + 15 \ \Rightarrow\ 3x = -27 \]
\[ \frac{3x}{3} = \frac{-27}{3} \ \Rightarrow\ x = -9 \]
\[ 3\cdot(-9) - 15 = -27 - 15 = -42 \checkmark \]

Trying values of x in 3x − 15 = −42

x	3x − 15	Equals -42?
-11	-48	no
-10	-45	no
-9	-42	yes
-8	-39	no
-7	-36	no

Questions

What is x in 3x − 15 = −42?

The solution of 3x − 15 = −42 is x = -9.

How do you solve 3x − 15 = −42?

Add 15 to both sides to get 3x = -27, then divide both sides by 3 to get x = -9.

How do I check the answer?

Replace x by -9: 3(-9) − 15 = -42, which is -42.

Solve 3x − 15 = −42

How to work it out

Trying values of x in 3x − 15 = −42

Questions

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