Solve 4x − 15 = 21

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

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

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The lines y = 4x − 15 and y = 21 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.
$4x - 15 = 21$
2
Add 15 to both sides:
$4x - 15 + 15 = 21 + 15 \ \Rightarrow\ 4x = 36$
3
Divide both sides by 4:
$\frac{4x}{4} = \frac{36}{4} \ \Rightarrow\ x = 9$
4
Check: put x = 9 back into the equation.
$4\cdot9 - 15 = 36 - 15 = 21 \checkmark$

LaTeX source of the formulas on this page

\[ 4x - 15 = 21\ \Rightarrow\ x = 9 \]
\[ 4x - 15 = 21 \]
\[ 4x - 15 + 15 = 21 + 15 \ \Rightarrow\ 4x = 36 \]
\[ \frac{4x}{4} = \frac{36}{4} \ \Rightarrow\ x = 9 \]
\[ 4\cdot9 - 15 = 36 - 15 = 21 \checkmark \]

Trying values of x in 4x − 15 = 21

x	4x − 15	Equals 21?
7	13	no
8	17	no
9	21	yes
10	25	no
11	29	no

Questions

What is x in 4x − 15 = 21?

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

How do you solve 4x − 15 = 21?

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

How do I check the answer?

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

Solve 4x − 15 = 21

How to work it out

Trying values of x in 4x − 15 = 21

Questions

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