Solve 5x − 4 = 21

The solution of 5x − 4 = 21 is x = 5.

5x - 4 = 21\ \Rightarrow\ x = 5

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The lines y = 5x − 4 and y = 21 meet at x = 5. 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.
$5x - 4 = 21$
2
Add 4 to both sides:
$5x - 4 + 4 = 21 + 4 \ \Rightarrow\ 5x = 25$
3
Divide both sides by 5:
$\frac{5x}{5} = \frac{25}{5} \ \Rightarrow\ x = 5$
4
Check: put x = 5 back into the equation.
$5\cdot5 - 4 = 25 - 4 = 21 \checkmark$

LaTeX source of the formulas on this page

\[ 5x - 4 = 21\ \Rightarrow\ x = 5 \]
\[ 5x - 4 = 21 \]
\[ 5x - 4 + 4 = 21 + 4 \ \Rightarrow\ 5x = 25 \]
\[ \frac{5x}{5} = \frac{25}{5} \ \Rightarrow\ x = 5 \]
\[ 5\cdot5 - 4 = 25 - 4 = 21 \checkmark \]

Trying values of x in 5x − 4 = 21

x	5x − 4	Equals 21?
3	11	no
4	16	no
5	21	yes
6	26	no
7	31	no

Questions

What is x in 5x − 4 = 21?

The solution of 5x − 4 = 21 is x = 5.

How do you solve 5x − 4 = 21?

Add 4 to both sides to get 5x = 25, then divide both sides by 5 to get x = 5.

How do I check the answer?

Replace x by 5: 5(5) − 4 = 21, which is 21.

Solve 5x − 4 = 21

How to work it out

Trying values of x in 5x − 4 = 21

Questions

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