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 subtraction, then the multiplication.
$12x - 15 = 21$
2
Add 15 to 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.
$12\cdot3 - 15 = 36 - 15 = 21 \checkmark$

LaTeX source of the formulas on this page

\[ 12x - 15 = 21\ \Rightarrow\ x = 3 \]
\[ 12x - 15 = 21 \]
\[ 12x - 15 + 15 = 21 + 15 \ \Rightarrow\ 12x = 36 \]
\[ \frac{12x}{12} = \frac{36}{12} \ \Rightarrow\ x = 3 \]
\[ 12\cdot3 - 15 = 36 - 15 = 21 \checkmark \]

Trying values of x in 12x − 15 = 21

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

Add 15 to 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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