Solve 12x + 15 = −21

The solution of 12x + 15 = −21 is x = -3.

12x + 15 = -21\ \Rightarrow\ x = -3

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The lines y = 12x + 15 and y = -21 meet at x = -3. 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 addition, then the multiplication.
$12x + 15 = -21$
2
Subtract 15 from both sides:
$12x + 15 - 15 = -21 - 15 \ \Rightarrow\ 12x = -36$
3
Divide both sides by 12:
$\frac{12x}{12} = \frac{-36}{12} \ \Rightarrow\ x = -3$
4
Check: put x = -3 back into the equation.
$12\cdot(-3) + 15 = -36 + 15 = -21 \checkmark$

LaTeX source of the formulas on this page

\[ 12x + 15 = -21\ \Rightarrow\ x = -3 \]
\[ 12x + 15 = -21 \]
\[ 12x + 15 - 15 = -21 - 15 \ \Rightarrow\ 12x = -36 \]
\[ \frac{12x}{12} = \frac{-36}{12} \ \Rightarrow\ x = -3 \]
\[ 12\cdot(-3) + 15 = -36 + 15 = -21 \checkmark \]

Trying values of x in 12x + 15 = −21

x	12x + 15	Equals -21?
-5	-45	no
-4	-33	no
-3	-21	yes
-2	-9	no
-1	3	no

Questions

What is x in 12x + 15 = −21?

The solution of 12x + 15 = −21 is x = -3.

How do you solve 12x + 15 = −21?

Subtract 15 from both sides to get 12x = -36, then divide both sides by 12 to get x = -3.

How do I check the answer?

Replace x by -3: 12(-3) + 15 = -21, which is -21.

Solve 12x + 15 = −21

How to work it out

Trying values of x in 12x + 15 = −21

Questions

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