Solve 4x − 7 = 21

The solution of 4x − 7 = 21 is x = 7.

4x - 7 = 21\ \Rightarrow\ x = 7

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The lines y = 4x − 7 and y = 21 meet at x = 7. 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.
$4x - 7 = 21$
2
Add 7 to both sides:
$4x - 7 + 7 = 21 + 7 \ \Rightarrow\ 4x = 28$
3
Divide both sides by 4:
$\frac{4x}{4} = \frac{28}{4} \ \Rightarrow\ x = 7$
4
Check: put x = 7 back into the equation.
$4\cdot7 - 7 = 28 - 7 = 21 \checkmark$

LaTeX source of the formulas on this page

\[ 4x - 7 = 21\ \Rightarrow\ x = 7 \]
\[ 4x - 7 = 21 \]
\[ 4x - 7 + 7 = 21 + 7 \ \Rightarrow\ 4x = 28 \]
\[ \frac{4x}{4} = \frac{28}{4} \ \Rightarrow\ x = 7 \]
\[ 4\cdot7 - 7 = 28 - 7 = 21 \checkmark \]

Trying values of x in 4x − 7 = 21

x	4x − 7	Equals 21?
5	13	no
6	17	no
7	21	yes
8	25	no
9	29	no

Questions

What is x in 4x − 7 = 21?

The solution of 4x − 7 = 21 is x = 7.

How do you solve 4x − 7 = 21?

Add 7 to both sides to get 4x = 28, then divide both sides by 4 to get x = 7.

How do I check the answer?

Replace x by 7: 4(7) − 7 = 21, which is 21.

Solve 4x − 7 = 21

How to work it out

Trying values of x in 4x − 7 = 21

Questions

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