Solve 10x − 9 = 21

The solution of 10x − 9 = 21 is x = 3.

10x - 9 = 21\ \Rightarrow\ x = 3

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The lines y = 10x − 9 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.
$10x - 9 = 21$
2
Add 9 to both sides:
$10x - 9 + 9 = 21 + 9 \ \Rightarrow\ 10x = 30$
3
Divide both sides by 10:
$\frac{10x}{10} = \frac{30}{10} \ \Rightarrow\ x = 3$
4
Check: put x = 3 back into the equation.
$10\cdot3 - 9 = 30 - 9 = 21 \checkmark$

LaTeX source of the formulas on this page

\[ 10x - 9 = 21\ \Rightarrow\ x = 3 \]
\[ 10x - 9 = 21 \]
\[ 10x - 9 + 9 = 21 + 9 \ \Rightarrow\ 10x = 30 \]
\[ \frac{10x}{10} = \frac{30}{10} \ \Rightarrow\ x = 3 \]
\[ 10\cdot3 - 9 = 30 - 9 = 21 \checkmark \]

Trying values of x in 10x − 9 = 21

x	10x − 9	Equals 21?
1	1	no
2	11	no
3	21	yes
4	31	no
5	41	no

Questions

What is x in 10x − 9 = 21?

The solution of 10x − 9 = 21 is x = 3.

How do you solve 10x − 9 = 21?

Add 9 to both sides to get 10x = 30, then divide both sides by 10 to get x = 3.

How do I check the answer?

Replace x by 3: 10(3) − 9 = 21, which is 21.

Solve 10x − 9 = 21

How to work it out

Trying values of x in 10x − 9 = 21

Questions

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