Solve 9x − 15 = 21

The solution of 9x − 15 = 21 is x = 4.

9x - 15 = 21\ \Rightarrow\ x = 4

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The lines y = 9x − 15 and y = 21 meet at x = 4. 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.
$9x - 15 = 21$
2
Add 15 to both sides:
$9x - 15 + 15 = 21 + 15 \ \Rightarrow\ 9x = 36$
3
Divide both sides by 9:
$\frac{9x}{9} = \frac{36}{9} \ \Rightarrow\ x = 4$
4
Check: put x = 4 back into the equation.
$9\cdot4 - 15 = 36 - 15 = 21 \checkmark$

LaTeX source of the formulas on this page

\[ 9x - 15 = 21\ \Rightarrow\ x = 4 \]
\[ 9x - 15 = 21 \]
\[ 9x - 15 + 15 = 21 + 15 \ \Rightarrow\ 9x = 36 \]
\[ \frac{9x}{9} = \frac{36}{9} \ \Rightarrow\ x = 4 \]
\[ 9\cdot4 - 15 = 36 - 15 = 21 \checkmark \]

Trying values of x in 9x − 15 = 21

x	9x − 15	Equals 21?
2	3	no
3	12	no
4	21	yes
5	30	no
6	39	no

Questions

What is x in 9x − 15 = 21?

The solution of 9x − 15 = 21 is x = 4.

How do you solve 9x − 15 = 21?

Add 15 to both sides to get 9x = 36, then divide both sides by 9 to get x = 4.

How do I check the answer?

Replace x by 4: 9(4) − 15 = 21, which is 21.

Solve 9x − 15 = 21

How to work it out

Trying values of x in 9x − 15 = 21

Questions

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